First off, I would steer clear of most "we can speed up your computer" applications. Most of them just get you to install their "free" version, which typically claims to scan and show you whats wrong. Then you pay to fix them. The other versions may appear to be completely free, but virtually none of them will do anything more than quite possibly trash your computer.
My recommendation is to use a program (I know goes against what I said above, should have mentioned ONLY use if someone you trust recommends... and you trust me, right?!?!) Anyway, there is an app called CCleaner (Crap Cleaner) from Piriform (http://piriform.com) download that, install it, (free for personal use) and then have it check for problems.
A couple comments here, you might want to uncheck in the overall check the options for cookies and web history (unless you don't mind having to log back in and don't care about sites you've visited recently). Then you can go to the Registry tab, scan and fix all it finds, but MAKE SURE you save a backup, on the off chance it makes something break you can just restore it from the backup by double clicking the backed up file.
Finally, under the Tools tab, first use the Uninstall to remove any programs you no longer use (again, be wary of what you are removing, make certain you don't use it). And finally click the Startup button and disable any applications from startup that you don't need (you can delete them later, but if you disable and something breaks (like a camera is no longer detected when plugged in) you can re-enable it. Common ones to disable are those having to do with iTunes/Apple, Adobe, printer "tools", etc.
Hope that gets you started, make sure you are also running current anti-virus software (I recommend Microsoft Security Essentials right from the Microsoft website). I've seen quite a few computers with a significant speed increase by removing McAfee or Norton anti-virus and replacing with Microsoft. Those others seem to really put a hamper on system performance.
Greg Hicks
Tekamba Computers, LLC
http://tekamba.com