'Disk Cleanup' is the tool to call on if you want to perform a general cleanup of unnecessary files or if you are low on disk space.
To run 'Disk Cleanup', click on 'Start', 'All Programs', 'Accessories', 'System Tools' and then 'Disk Cleanup'.
Here you can then select from a list file categories, the general ones being the 'Recycle Bin', 'Temporary Files' and 'Temporary Internet Files'. Each category indicates the space that can be freed up.
Warning: Make sure that you check the category's correctly and read the description for what each one does, for example you may not want to delete files from the Recycle Bin, that you might yet want to restore.