There are lots of things that could cause your PC to slow down, but I'll try to address the most common problems and how to fix them. Also, while spyware and viruses are unusual in the Mac or Linux world, the recommendations I make can be applied to them just as easily.
Spyware is malicious software that lurks on your machine doing all kinds of annoying things, but the one thing we're interested in right now is that it can really slow everything down! There are several good tools available for free on the Internet that can take care of most spyware, but the one that I come back to time and again is Malwarebytes Anti-Malware. Simply install the software, make sure it's updated, and tell it to scan your machine. A quick scan is helpful, but to be thorough, a full scan is the way to go. Be warned: a full scan can take an hour or more.
There are several other good anti-spyware tools available, including Adaware and Spybot Search and Destroy, so if Malwarebytes doesn't clear up your problem, try a different vendor.
Assuming there are no spyware or virus problems on your machine, the biggest bang for your buck in terms of improving performance on your machine will be to increase RAM. It's easy, simply go to Crucial's website (crucial.com) and enter your machine model information and it will tell you how much memory your machine supports, exactly what chips to buy to match your machine, and several different options for cost. Some machines require RAM chips to be installed in pairs, and Crucial even warns you about that.
To see how much RAM you currently have, right-click on "My Computer" (or just "Computer" in Windows 7) and then click "Properties" and a page will come up and tell you about your system.
Installing RAM is trivial if you're not intimidated by opening up the case on your machine (and you shouldn't be!). Static electricity is the most likely cause of damage to a machine you're working on, so make sure you're not on carpet and ground yourself on something metal before digging into the machine.
3) Defragment your hard drive
Hard drive fragmentation is pretty esoteric, but suffice it to say that Windows has a tendency to write pieces of files all over your hard drive. This causes problems in performance when you want to access those files later because the hard drive has to look all over the disk and reassemble the file from pieces before presenting it to you. In order to fix this problem, you should run the built in Disk Defragmenter program located under: Start -> All Programs -> Accessories -> System Tools.
The built in tool works fairly well, but I recommend you try another free program, Defraggler. Defraggler appears to be significantly faster than the built in tool, plus it will work if you happen to be below 10% available hard drive space on your machine.
4) Get rid of all the toolbars!
Many home users end up with two, three, four, or even more toolbars in Internet Explorer. This is definitely a case of "more is not better"! Pick one and make sure it is a reliable one like Google's toolbar and then remove all the rest of them. You can disable toolbars by opening up Internet Explorer and selecting Tools -> Manage Add-ons. Better still would be to uninstall them entirely and you can do that going Start -> Control Panel -> Add/Remove Programs and selecting the toolbars you don't want and uninstalling them.
5)Install or update your antivirus
I'm fairly agnostic when it comes to antivirus vendors, but if you don't have anything, you need to get something right now. Free antivirus products are available from AVG, Avast, ClamWin and others, and excellent commercial products are available from Symantec, Trend Micro, McAfee and more. Get one, keep it updated, and let it scan your machine regularly. If you have an older machine, don't go for a bundled product that also includes firewall/anti-spam/anti-spyware/etc. because it will definitely bog your machine down.