If your PC originally had Windows XP on it, it may not be entirely compatible for Windows Vista to run properly on it.
Microsoft has a free online service called the Vista Upgrade Advisor which you are supposed to use before upgrading, but it can still be used after the upgrade to find out what incompatibilities are probably causing the problems you're having. Go here: http://www.microsoft.com/india/windows/products/windowsvista/buyorupgrade/upgradeadvisor.mspx
It's also worth noting that Windows Vista uses considerably more RAM than XP. Vista requires at least 2GB RAM for it to run smoothly, even though Microsoft says it will run on less, in practical terms there claim is way too optimistic.