My computers hard drive C: is almost full only 9% free. Its running very slow, I tried to defrag but it says I need at least 15% free.

I don't understand what is filling it up, we don't have much stuff installed, we do have loads of pictures though, could this be filling it up?

How do I find out what is taking up the space and how do I remove unwanted files etc?

I am using windows XP.

Thank you.

Here is a nice little scanner:

Download Disc Space Reporter 1.0 - Disc Space Reporter - Disk space information tool - Softpedia

I'd suggest getting an external drive and move your pictures over to it.

Before you defrag, you may want to do some clean-up.
Start by running the windows clean-up utility: double click My Computer, right click on the C drive icon, click Properties, click Disk Cleanup. Based on your stats, this process may take awhile. After it is finished you can try the Dfrag again.

There are also some third party applications that do a good job, one of theses is CCleaner (AKA Crap Cleaner). I use it as part of my monthly clean-up and have never had a problem. However, you may want to read about the features & settings before using.