Can I use a iMac Late 2009 or Mid 2010 as a screen for regular PC

Yes an iMac will work as a screen for a regular PC, but only with 27" models of Late 2009, 2010 iMacs. If those conditions are met then the iMac supports what is called Target Display Mode, aka using your iMac as a display.

Also you will need a Display port to mini display port cable, they do exist as sold on Monoprice and at other fine online retail locations.

For more information on using your iMac in Target Display mode see Using a 27-inch iMac (Late 2009 or Mid 2010) as an external display

To use an iMac (27-inch, Mid 2010) or iMac (27-inch, Late 2009) in Target Display mode with another computer as the source:

1. Make sure both computers are turned on and awake.

2. Connect a male-to-male Mini DisplayPort cable to the Mini DisplayPort on each computer. The 27-inch iMac will enter Target Display mode and display content from the source computer. Note: If you are connecting two 27-inch iMacs, connect a Mini DisplayPort cable to each computer and press Command + F2 on the 27-inch iMac keyboard that you will use as an external display.

3. To leave Target Display mode, press Command + F2 on the keyboard of the 27-inch iMac that is in Target Display mode. To return to Target Display mode, press Command + F2 again.