If you're really wanting to keep this machine I would advise you to reinstall the operating system. If the system is 4-5 years old I would hesitate before putting on XP though. Yes the computer would probably handle it assuming you have sufficient RAM but the hardware may not be sufficient enough to run "nicely". In other words it would probably be slow. I would consider using 2000.
However, if you don't want to do this I recommend you do the following.
First, defragment your Hard Drive. Over time the information becomes spread out on your hard drive, fragmented, and for your hard drive to find the information it has to search in many different places. So in essence by defragmenting your hard drive you are basically just putting the information back in order.
If you are using windows you can
Click Start
Go to all Programs
Go to Accessories
Go to System Tools
Click Disk Defragmenter
This will bring up a program all you have to do is click Defragment
Don't be alarmed if this takes a long time. I've had an older machine take a day or two before just because its so fragmented.
Another thing you should do, if you haven't already, is to download some Anti-virus and Spy-ware Removers. I personally like "AVG Anti-virus" "Spybot Search and Destroy" and "Ad-aware 2007" you can find all of these at
Free Software Downloads and Software Reviews - Download.com and best of all they're all free.