If you go onto the 'Performance Information and Tools' screen you get information on the performance of various parts of your computer.
You can find this screen by going onto Start/Control Panel/Performance Information and Tools or just do a search on your computer for it.
It'll show a sub-score for things like your Processor, Hard Disk and Graphics etc. so you can see what is holding back your system, as well as an overall score for the whole system (base score). The base score is whatever your lowest sub-score is.
According to windows help and support "A computer with a base score of 4 or 5 is able to run all new features of Windows Vista with full functionality, and it is able to support high-end, graphics-intensive experiences, such as multiplayer and 3‑D gaming"
For example, I "can" play on 3D but it runs slowly and the picture jumps instead of being smooth. When I looked at this information my graphics and gaming graphics sub-scores were 3.1 and 3.0 whereas the others ranged from 4.7 to 5.5. So, if I wanted to make my game run smoother I'd have to upgrade my graphics card.
Normally I don't reply to threads like this but I get annoyed when nobody helps me out so I figured I can't expect people to help me if I don't help them sometimes.
Hopefully this'll help you figure out what you need to do.