Don't know what system your on or what you have lost but...
OK, if I were you. You need to scan disk first. If that stalls, you have issues, just keep restarting. If that doesnt work, try and get onto your desktop (cancel scan disk), perform a spybot check (great ones at download.com - spybot search and destroy for a basic app, if you need more thorough checks come back for more info.).
Restart your system. Try scan disk again. If that works, great...
Moving on to defrag... best way is, ctrl alt dlt, shut down everything apart from what it takes to keep your system running, it would help if I knew what windows you were using but generally keep open Explorer and Systray and your fine, shut everything else down, don't have ANY internet programmes running in the background especially. Just don't touch the bugger while it'd defragging... (do you even need to defrag???)