Recurring Disk Space Problem

Hey Experts,

I've got an HP Mini 110 running XP Home. I use it to read email, do some wordprocessing and watch movies at hulu and amazon. My browsing history gets cleared fairly regularly. Every couple of months or so, I do a defrag and run the utility that cleans up the hard drive. Microsoft essentials says that I have no viruses.

And yet, the mini is running slow and when I went to defrag today, I got a message that I don't have enough free disk space to run the utility. This seems to happen to me on all of my laptops. I am obsessive about removing programs I don't use, don't store music or photos on the mini. What am I overlooking?

Please help with this. I'm not an expert by any means, just someone who doesn't want to lose this computer.
Thanks for your reply. My question is, why do I always end up with this problem of not enough disk space when I don't store anything locally? I understand that I need 15% free disk space, but there is nothing to remove that I know of.