There are some freeware and commercial retail products you can try.
Ccrap Cleaner is a good freeware app that fixes registry entries, cleans out cache, temp files etc.
HIjack this is a great free tool if suspect being hijacked i.e. slowdowns due to adware, spyware etc.
Using
DISK CLEANUP, ccrap cleaner along with Windows defrag will do a decent job.
Tuneup Utilities, System Mechanic, Fix It Utilites 9 or Norton SystemWorks are a few popular retail all-in-one suites.
*Can be downloaded as trial versions before consider buying. Just want tweaking tips? Try this site:
http://www.blackviper.com/If you suspect major probs and rarely maintained your computer in the past then sometimes a clean format is the faster, efficient solution. Otherwise, you will have to dig deeper for malware, spyware, virii, trojans, worms etc that may be infecting your computer. A good firewall, anti-virus and on-demand scanner for spyware/malware/adware can't be stressed enough. Just using an alternative browser like Firefox can reduce chance of being infected or being hijacked.
Good luck!