Can I legally downgrade from Win 7 64 bits to Win 7 32 bits? [duplicate]

Yes for retail Windows 7 licenses.

When you buy the retail DVDs Microsoft gives you both versions, 32 and 64. As long as you only install and license one of them at a time, Microsoft doesn't really seem to care which you use. I have downgraded one system with no issues - though you will have to backup and reinstall everything - but you knew that.

For OEM licenses, at the time you purchase an OEM copy of Windows 7 to be preinstalled on a new PC, you must choose either 32-bit or 64-bit Windows. Your agreement with the OEM determines whether you can switch to a different version; some PC makers support only a single version with specific PC models and will not allow you to switch from 32-bit to 64-bit (or vice versa) after purchase. Source

What hotei says is completely correct and answers your question but I would just like to add some points regarding 32 and 64-bit.

The instability may not be due to Windows 7 64-bit, and could instead be due to the mash of 3rd party software installed by the manufacturer of your laptop. The manufacturers sometimes put a lot less care into their software than they do their hardware as can be found at The HP CPU Assassin SU blog post.

There should be no real reason to downgrade from 64-bit and if anything 64-bit should be more stable due to the requirement of drivers to be properly validated prior to being installed.

If you have 4GB or more of memory then 32-bit will cripple your laptop and you will end up with less memory available for the system to use.

My first recommendation would be to reinstall Windows 7 64-bit from an install CD and only install the drivers from the manufacturer that are actually required to make all the devices work, don't install their "helper" multimedia software or anything else, and see if that is more to your liking and is more stable.