The following utilities will keep your system running nice and smooth..:
Window Washer 5.5
Spy Sweeper 3.0
Registry Mechanic 3.0
Check the Task Manager for any process that doesn't serve a purpose. If you are unsure about one of the processes running, you can find out what it is here
Also check what is starting up with Windows. Under the "Run" command, type "msconfig.exe" (Windows XP only).
Don't forget to run a defrag once a month (or depending on how frequent the PC is used).
Good luck!...! And I hope this helps!
P.S.: What was mentioned above is the only alternative to reformating the HDD. However, reformating will be much more effective.