kids, 2 years ..... oh boy do you need to rebuild the machine.
(I've done this since windows 3 - every 12-18 months)
the rubbish that accumulates can't be cleared any other way .... and if your kids are that certain age ... they'll be swapping tunes, progs, downloading - all the stuff kids do.
If their bedrooms are messy ... think how bad the PC is likely to be when they share!!!
problem is .... the others are right .... it's a rotten job which will likely result in you loosing all your stuff ...... unless you prepair for the job.
before you get bogged down ... try CC Cleaner
http://www.ccleaner.com/download
despite the paypal links - it's free
if that improves things .... well stop there
if not (and I suspect it won't)
first ... make sure you have all the CDs that you need to rebuild (windows OS and drivers - or a restore disc).
without them .... don't even think about it
Then backup everything you (or the kids) can't do without.
then .... do the deed - you will feel the benefit