Cannot launch task manager? I faced the same problem today, from a laptop and a computer at the same time, in the same room. One belongs to a friend who I helped today (recovering DVDs, backing up, copying music to CD, etc) and the laptop is my dad's. In both cases, I figured out that there's one primary cause to why they're slow.
Overtaxed HDD. The more processes trying to access the HDD, the longer the queue gets, the more everything slows down. This gets especially true if you've not wiped the damn thing in the last 12 months and they download all sorts of random stuff and don't read everything before they click and stuff. It's not as much about the crapware (though that does compound the problem) as Windows trying to do too many things at once because the registry is too big/cluttered. Fragmented system files don't help either. That's why I tell people to wipe their OS drive every year to start fresh.
