Simple methods that may fix your computer
In case you were wondering about how to fix your own Windows box, here are some suggestions:
Download and install all of your Windows updates, in general updates are good, sometimes an update is released that hurts or doesn't really help.
Get an antivirus package and run it, I like the free version of AVG
Get a registry cleaner, I like ccleaner
clean the registry, clean your temp files etc.
Malware remover, I like spybot
Windows has it's own malware remover now, so it's your choice
Defrag your harddrive
The defrag in windows works ok
Uninstall programs you never use, like all those default programs Dell etc gave you
Still Slow? Upgrade your RAM
Still Slow? Reinstall Windows or...
Maybe you need a better motherboard/cpu, ie. time to get a new computer. Harddrive upgrades can work as well, but when you get to this point a new computer is probably the best solution.
If your computer is over 4 years old, it may be time to upgrade.
I may be missing some stuff, and I'll see if I can post some screencasts and links to these free but worthwhile programs, in general google will find you links to download them :)