Why Is My PC Sooooo Slow?

Anyone got any suggestions to speed this machine up?  Even I can run faster.

I run AVG every week and it comes up blank.  I've used the disc clean up feature quite often, and I can't blame my internet connection cos the kids laptops run just fine.  It's not that old and has only started running slow in the last few weeks, and that includes starting up, which takes ages.

Any suggestions?  Thanks in anticipation

If you can pin down when the problems started then I'd suggest undoing whatever  you did around that time, eg remove updates, hardware or programmes added then, one at a time to see if there's a noticeable improvement.
If it's less obvious than that Task Manager can (sometimes) give a clue to a specific process causing problems. Also as Les suggests try removing stuff from startup.

In the past I've not tinkered too much if a PC gradually slows down after 12 - 18 months. I always favoured rebuilding from scratch especially Windows XP & Vista, although Windows7 seems OK so far.
This can be a daunting step if you've not done it before and requires that you have all the CDs/DVDs with OS, software & data available.