Well, the matter of having a slow OS is a multi-headed dragon. You really need lots of experience in the field to detect and solve all the little problems that can eat away at your system speed. I'll give you a few pointers.
1. Check to make sure you're not wasting memory by starting up tons of programs you don't need when windows starts. You can use the msconfig utility's startup tab to do this. Common examples are Adobe updaters, iTunes, IM clients, toolbars, etc.
2. Clean out your registry with a registry cleaning program. Just do a google search and you should find a few.
3. Uninstall programs you don't need, especially adware types like toolbars and other downloaded applications.
4. Run a disk defrag.