How can you tell when it's time to get a new computer before it actually dies?

I have a Dell Dimension 5100 that I bought in summer of '05. It has had a pretty good run but because of my very very limited knowledge of computers and of my use (really only use it for email, light games such as pogo.com, youtube/hulu videos, social networking, music/photo storage etc. nothing heavy) I have done no upgrades to the inside and am not sure if it's time to just go ahead and get a new one or stick with what I have for a little longer.

For the last year or so I have noticed it slowing down quite a bit. I have no viruses and have not recently installed any big programs. It can read CD's just fine but has not been able to read DVD's for the last 3 years, that never bothered me though as I was not a heavy user of that in the first place so I never got it fixed. It's the worst when I'm on the internet and ask my computer to do more than a couple of things at one time. There's frequent browser crashes (with multiple browsers), sometimes freezing that forces me to reset the whole computer, slow when loading web pages. My itunes is painfully slow to open up. The fans are a lot louder than they used to be, especially when I have itunes open. It seems fine when opening up simpler programs (word, solitaire, My Pictures etc.)

Is my computer on it's last legs or does it just need some tender loving care from a knowledgeable computer tech?