I used to use a sheet of sand paper with a bit of industrial cleaner for good measure.. until I heard of the method Steve mentioned. There are lots of tools to do things such as clean up registry or remove unnecessary files but really you have to manage it as you go along. After a certain point, these programs make minimal difference and a format definitely starts to become the best option. Defragmenting the drive sometimes helps though. You could take a peek at your startup programs too. As Steve said, comp stats would be helpful.
This is assuming that your computer has and can run faster, too. If it's always been this sluggish or close to, an upgrade is possibly the only real option.
sounds like it could be malware but let's not jump the gun.
I've been using this program called "easycleaner" for a while now I'd recommend that, it will help you clean out the registry and delete double entries, and entries pointing to nowhere. This sped up my computer drastically when it started to run slowly.
In the end you may have freed up a significant amount of disk space too depending on how many invalid entries there are in your registry.
Ccleaner works well for cleaning the HDD, and does an ok job with registry cleaning. For defrag download auslogic. A hint on auslogic, before running change the CPU setting to highest.
I also use Revo uninstaller because it has a junk file cleaner function, plus you can easily change what programs run at start.
Seriously speaking, its a good idea to dump everything off your hard drive every 6-12 months. I'm talking a complete reformat. Save vital info onto a USB and then wipe the rest away. Its what I do. Also, just like cars, on computers parts do go bad. They should be replaced every so often to keep your computer working and up to date.
If you're paranoid like me, you might also want to zero-out the drive with multiple passes. Makes it more difficult for the Feds to pull off evidence of thoughtcrime.
Before you go and do that, though, I'd check to see if you are running two different antivirus programs at the same time. That can cause huge performance issues, and it's a problem I often see on computers that came with Norton and the owner just installed their favorite Antivirus on top of it.