Usual thing, reboot into safe mode (Press f8 whilst the PC is booting up, you may need to keep pressing it to 'catch it', you will get a menu, choose safe mode, not safe mode with command line)
It will take a bit longer than normal to boot and will look terrible as it defaults to 640x480 resolution with only 16 colours. Run Defrag from in there also worth running scandisk as well jsut to be sure.You will not have access to CD drives, internet or anything else as it doesn't load any devices beyond what is absolutely necessary.
Or....(Ian putting his usual plug in) use an OS that doesn't need defragging
