How could i use an iMac as secondary monitor for a Windows 7 laptop

Thunderbolt-equipped iMacs prior to iMac (Retina 5K, 27-inch, Late 2014) (see this document), like your iMac (Mid 2011), support Target Display mode:

Target Display Mode lets you use your iMac as the external display for another, “primary” computer.

Thunderbolt-only solution

NOTE: This won't work for iMac (Retina 5K, 27-inch, Late 2014) and later iMac models as they don't support Target Display Mode.

If you happen to own a laptop with a Thunderbolt port follow these simple steps to extend your display (Thunderbolt to Thunderbolt cable required) (from KB PH4469):

1. Connect the Thunderbolt cable to the Thunderbolt ports on each computer.
2. Make sure the iMac and the primary computer are turned on and awake.
3. Press ⌘F2 on the keyboard of the iMac.

(As a side note: Older iMacs with Mini DisplayPort also support Target Display Mode. The steps to configure and enable Target Display mode are very similar.).

A comment on DVI to Thunderbolt adaptors

You can't use Mini DisplayPort to DVI or VGA adaptors:

to connect a computer with a DVI/VGA port to a Thunderbolt display (from this thread):

I can confirm that Mini DisplayPort to DVI converter works ONLY from a Mini DisplayPort computer port to a DVI display. The reverse direction is not supported by those converters,

There is some hope, though, as the post goes on to say:

but there are other much more expensive ones that will convert DVI computer ports to Mini DisplayPort display.

The poster is probably refering to a product mentioned earlier in the thread: the Kanex C247D Single-Link DVI to Mini DisplayPort Converter, advertised like this:

Connect a (...) LED Cinema Display to your DVI-equipped MacBook Pro, Mac Mini, or PC with this (...) Kanex solution.

However, opinions diverge on whether it will work with a Thunderbolt display at all:

• I am sure that Kanex don't do anything that will help you no matter how expensive it may be.

• As far as I am aware, and I have done a lot of reading about this, you can connect an older display to the new Thunderbolt port of a new MAC but you CANNOT connect the Thunderbolt display to anything other than a MAC with a Thunderbolt port. The Kanex thing works from a Thunderbolt port into an older display, DVI or whatever but you cannot convert MAC DVI into Thunderbolt......I think

I've noticed that the LED Cinema Display is listed as a Mini DisplayPort display (from http://support.apple.com/kb/SP502):

so the adaptor may indeed only work with Mini DisplayPort displays, not with Thunderbolt displays, and as far as I can tell, your iMac will act like a Thunderbolt display.

It's up to you to try it, although $129.90 (as of this writing) is a steep price to pay for the Kanex adaptor just to test it.

Software-based solution

If the solutions described above aren't feasible or cost-effective, you can use several software solutions. One is Air Display:

Use your iPad, iPhone, Android, Mac or Windows PC as a second (or third) monitor with no messy cables or wires

There is a free trial version here.

Note: Windows 7 Starter edition is not supported.

In addition to the above, other software solutions are Jinx, who offers a product called ScreenRecycler. Couple this with a VNC client like TightVNC or JollysFastVNC (from Jinx), you pay once for the main computer, and can use any number (one at a time) of other computers as a remote display. This software is currently in Beta, and I found that after 5-10 minutes of use, the remote display slowed and became very choppy. I don't know if this was because my main computer is running OS X 10.11 Beta, but I suspect it is just issues with the ScreenRecycler product. Also, the VNC Client does not detect the screen resolution of the host (remote) computer, and so does not allow you to take full screen unless it happens to be one of the predefined resolutions.

I found Air Display to work very well with acceptable performance, using 2 laptops (1 Windows, 1 Mac) over WiFi 802.11n. Mac was primary, Windows was remote monitor. However, you have to pay for each machine (1 at a time) that you use as a remote monitor - opposite of how ScreenRecycler is licensed. Air Display also used the host computer's screen resolution nicely, but at times motion became choppy.

Finally, there is MaxiVista - this one is one-way, only uses Mac, Windows or Linux as a remote monitor for Windows. I have not tested it, as what I wanted to do was use my Mac and extend to external monitors wirelessly, but the hardware is too old to support AirPlay to an Apple TV. MaxiVista runs from $49.95 to $129.95, which I find to be way too expensive, as the others are around $20. For the price of MaxiVista, you may as well just buy a physical monitor and skip the network bandwidth utilization and lagginess.

Incase anyone is interested, this is a bit late but it could be useful for anyone who searched and finds this page.

I had a 27" 2010 iMac and a pc with a DP (DisplayPort) connection so I just purchased a mDP (Mini DisplayPort) to DP cable from PBTech ($11.50 NZD). I don't know what connections you have on your laptop but all adapters I could find worked from mDP to VGA and not the other way around.

VGA carry analog signals which in terms of quality are one of the worst cables when connecting to an LED or similar screen. It's also probably why there are very little/any VGA to mDP adapters as they would be expensive and not too many people would purchase them. All sources I searched stated that it was either very difficult or impossible to go from an analog VGA to a digital mDP.

I did find something that should work for DVI but it is a bit pricey. You would need a Dual Link DVI to DP ($150 used - none selling new) and a DP to mDP ($7.53).

According to this Source VGA to DVI is possible but the cables required "are harder to locate and often cost upwards for $100" plus you would still the above DVI to mDP setup

tl:dr?

DP connection:

• Upwards of $8 to get a DP to mDP cable

DVI connection:

• ~$158 to get a DVI -> DP and a DP -> mDP

VGA connection:

• If you can find a cable/adapter expect to add on at least $100 on top of what you would pay for a DVI connection

Hope it helps and doesn't empty out your wallet.