Hello,,
There are a lot of things you can do, also alot of variables if you don't know what you are looking for or to fix exactly. Some things you can do:
Run Windows Cleanup
Defrag harddrive
Run some performance applications such as 'CCleaner'
Backup Registry and go through it and delete any useless entries (there are a few programs for this, I suggest you read up and don't just run the first one you see)
Check system temperatures
Check process', minimize running only whats necessary
I'm sure the list is much larger but those are a few places you can start, my favorite option? Backup, reload windows :)