The constant upgrade thing used to bug me about pcs, until I got myself into a bit of a groove. I got an AM2+ motherboard with the cheapest athlon64 cpu I could get and basic stuff for everything else. Now every few months I get something new, usually for around 100 bucks or less. Eventually the entire system will have been replaced but I did it in cheap little steps.
I keep my windows install very clean and it is fast. I take care of freinds and family members computers, and I can tell you there is a lot of crap that can slow your computer down. I don't blame the user, some of the toolbars and stuff sound really good, but they really aren't. Toolbars, adware, malware, viruses, all that stuff steals memory and cpu time.
Check your computer often or have it checked. Windows XP and 7 really are very fast, responsive Os's. If they aren't there is something wrong. And complaining doesn't fix it. Don't use a Pc that isn't working properly get it taken care of.