I suspect that you've got something running 'in the background' (such as anti-virus software) which is conflicting with the program you want to use.
Restart your PC, pressing F8 during the time when it's 'getting itself ready'. (i.e. before the Windows screen appears). That will take you into Safe Mode (without 'background' programs running). Your screen will look odd (because the video driver won't have loaded) but you can still access Start > Programs > Accessories > System Tools > Disk Cleanup.
After running it, restart your PC again to get out of Safe Mode.
Chris