1) Ram - the thinking memory. If you have XP 1 Gig is enough. If you have Vista or 7 you want, at least 2 gigs and 4 would be better. Windows 8 I would recommend 4 to 8 gigs
2) Look at what automatically runs when the computer starts. Lots of times there are programs that you didn't even remember putting on the machine. Others are programs you use on occasion but have no need to start every time Windows does. In the start menu type msconfig. It will have a tab called "start up" you can then go an uncheck whatever you don't need regularly.
3) Unused programs. Go into control panel and remove anything that you don't use.
4) Speed up the internet by going into internet options and clearing out cookies and history.
This list isn't a magic bullet but should perk things up a bit.