Vista was an abomination of an operating system. Most tech folks totally skipped it sticking with XP and going straight to win7.
Having said that Win7 does have pretty heavy system requirements, it's just they are more inline with current average system specs than vista had.
As claire said though, if the computer was once fast, there is no reason it should have gone slow unless you've installed new software which has higher requirements.
The slowness is most likely due to "stuff" that's been added as you surf the web or install programs. Some of this is virus/spyware but alot of it is just clutter, that get's installed when you install things like itunes or whatever.
If as claire suggests, it's no better after doing a virus check (, spyware check, and defrag, then one program i like to use is Glary Utilities although it's not the easiest to use you can have a go..
http://download.cnet.com/Glary-Utilities/3000-2094_4-10508531.html?part=dl-6280556&subj=dl&tag=buttonUnfortunately alot of commercial virus checkers are as much full of junk as the spyware they are supposed to stop, i hate stuff like norton and macafee, they really do stink and are the first thing i uninstall from any system. For a quick simple and safe virus protection just download microsoft security essentials, it's free and works fine, and is alot less bloated than the "commercial" software.
For specific anti spyware in addition to security essentials i like spybot seek and destroy, which is also free and works well.
As for glary utilities, if you run it got to optimise and improve, and run startup manager. It will give most likely a long list of all the programs that are startup up when windows starts. In truth most of these will be unnecessary junk. I currently have my list totally empty as they are simply not needed or helpful (to you). The problem is most of them keep coming back like the itunes helpers, each time you update the software so you need to keep removing/disabling them.
Basically just untick stuff and restart the system, if somethings not working re tick it. You can't really cause any damage. This should if nothing else speed up windows startup.
