I was *this* close to getting a new computer this weekend. Then I remembered some advice a friend of mine had given me that I had just recently passed on to others. If your computer is very slow as of late and it once performed very well, don't just blame Windows or a fragmented hard drive. Check the dust bunny quotient.
I tore my computer down last night and found: dust caked on the CPU cooling system, dust caked on a memory stick, dust all over the fans.
Dust on the fans makes them less efficient due to drag. Air moving over the blades of the fan is slowed, reducing the amount of hot air, and hence heat, which can be displaced. Displacing heat from your computer is essential to keeping the chips running in peak condition. Too much heat in the chips causes electron flow to literally slow down as resistance is increased.
Dust on the heat sink for the CPU also contributes. Not only can the fan not move air past the fins on the heat sink if dust is in the way, the dust acts as a blanket helping the heat sink to retain more heat.
Now this one I did not expect. Dust on my memory sticks. The fan for the CPU in my setup blows right on a memory module. Dust was caked up to the point that I suspect it was causing a short on the surface - just enough for the module to test bad on Boot. So, after cleaning it, I got half my Gig of memory back!
To clean my system, I used the soft brush attachment for my vacuum cleaner AFTER removing the components from the case. Trying to put a vacuum in your case could damage things, so be careful in there!