Your computer is likely not up to specifications.
Make sure you check before you buy games to see whether or not you can play them.
Just because you meet the minimum does not mean you can always play them easily. At least aim for having the recommended parts.
If upgrading is out of the question, making sure you have proper air flow, and no dust in your computer can go a long way.
A good investment is a heavy duty air compressor.
Works EXTREMELY well for cleaning computer parts.