I'm running on W7 and 4Gb of RAM. I've never cleared flash before a couple of weeks ago and never had problems. For me, clearing cache made absolutely no difference.
Have a look at what apps you have got running in the background and whether they need to be running, e.g. MSN, and loads of others who's names now excape me as I always do this when I first get a new PC. I have seen Norton Security grind machines to a halt... literally! I have had to start them up in safe mode, stop Nortonfrom starting on machine boot-up, then boot the machine up again. So, if you have Norton I would seriously think about getting that replaced with something less resource hungry.