About defragmenting your hard drive:
Defragging is only necessary if you write a lot of stuff to your hard drive and delete at a lot of stuff too.
If you use your PC mostly for web browsing, doing it once every 6 months is fine. If you are doing lots of work on your PC, AND it's EXTREMELY slow, then try it once a month.
HOWEVER, if things are really slow, try getting more RAM first and twiddle with your virtual memory settings. There may be an issue with setting the priority of your process threads, i.e. which jobs get done first.
Right click on your My Computer icon and pick Properties. Under the Advanced tab will be a button for Performance settings. Your default should say on the Visual Effects tab, "Let Windows choose what's best for my computer." Personally that's what I would do, but if you are suffering from performance problems, click the third option, "Adjust for best performance."
Next, pick the Advanced tab and make sure that the Processor Scheduling and Memory usage are set for PROGRAMS and not for background processing. After that, pick the Virtual Memory button an dyou can have the System manage your maximum size.
That should also help. Frequently it's about memory management and not having 10,000 open windows. Try only to run a few things at a time. Something that drives me nuts at work is folks who complain about system slowness and then show me a window full of streaming audio, multiple chat windows, 10 emails they're reading, 2 spreadsheets, etc. It's a mess. Then they ask me why Notepad won't open. I don't know what to tell them.
About memory leaks and spyware:
oh. and I forgot. TURN OFF YOUR PC. Older Windows systems had a memory leak that was well known. It's largely gone from XP, but periodically shut off your machine to really clear the cache out. I always leave my pc on. Replacing a fan is nothing compared to always cold starting your motherboard and having to replace that instead.
Also, sometimes if you're computer has been taken over by a virus or hackers, your PC will run very slowly because it's doing lots of background things as an 'owned/pwned' machine. Spyware removal will help.
About backups (very important!):
If you go and uninstall, reinstall Windows, make sure you take a back up of your hard drive.
I'm not kidding. As an IT professional, I've seen people do things without making a backup, that have literally made me and an entire dietary staff of a nursing home cry because they lost a week's worth of menu planning for people with special dietary needs.
BACK UP YOUR DATA IF YOU THINK IT'S IMPORTANT OR IF YOU ARE GOING TO MESS AROUND WITH YOUR OPERATING SYSTEM.