How to clean up your computer?

I want to clear a lot of space on my computer.
Do you have to do a system restore to get the best outcome?
what else can you do?
What’s the best free protection for your computer also?
Thanks 🙂
btw – I have a compaq, and windows xp
what is safe to uninstall

20 meg and I feel like my computer could need a little boost to speed up as well. It is a Compaq 3300+AMD Athlon 64 processor with 512 MB PC3200 DOR SDRram memory, 250 gb/7200 rpm. I have done defrag, utilities cleaners,start-up cleaning.

My friends computer has become infected with spyware or a virus. He is running Windows XP on a Compaq. Theres a little dialog box popping up constantly that says theres a virus on the computer. When you click on the box it redirects you to a spyware remover page. Obviously its an attempt to sell this product. How can I remove this and stop all these pop ups? I was thinking about manually cleaning the registry. How would I go about that? And what about those programs in the Add/Remove programs in control panel that just wont go away? I need to get this junk off his computer. What do I do? Without spending money that is.

I have already installed other cleaners like "Registry Optimizer", "Regseeker", and "Registry Bot" so far they all find different problems and don't seem to fix or remove any of them. My OS is windows 98 and my computer is a Compaq and it is a little on the older and slower side. Any and all help would really be appreciated.