Ok, depends how far you really want to go with this, but there are a few things you can do:
Firstly, unless it's what you've already used, go find yourself a copy of CCleaner. Cleans a load of unnecessary crap from your system, and should make a bit of difference at least.
Next, assess the software you have installed by going to control panel, then add or remove programs. Uninstall anything you don't need, and make sure there aren't more than one of the same type of software (e.g., are you running more than one antivirus program?) If you say your internet is running slow, remove ALL toolbars. Seriously, they really make a difference to your browsers speed.
And if you're not already, consider using Google Chrome / Mozilla Firefox as your browser. Waaaaay faster than IE, and less likely to let something in that'll rape your computer.
Another small modification to make would be to adjust the visual options. Do this by going to your desktop, right clicking anywhere, then selecting "personalise". Go through everything in there and untick anything that you don't really need (like transparent windows etc...)
Finally, go start>run then type in "msconfig". Hit enter, then go to the Startup tab. Uncheck anything that you don't need to start when you turn your computer on (most programs that aren't Microsoft, or your antivirus)
When you've finished on this tab, move to "Services" and likewise, remove anything you don't need. Things like programs that you don't need all the time and updaters for stuff you don't need updating constantly (again, most things except Microsoft software and antivirus.)
And of course, if it still runs slow as hell after all this, format. That's about all I can think of for now, but will add to it later if anything else springs to mind!
Good luck