[Question] Chkdisk and defrag...

Last night my system had a strange problem. It would randomly freeze entirely couldn't even get to task manager. I did all my scans and it turned up empty for malware, I checked to see if their were OS updates available, none, I even defraged it and before that Vista did a boot time chkdisk I decided to let it do it's thing. However all during this the PC would boot to desktop flawlessly so heres my question. What exactly does chkdisk do anyway? I don't know what fixed the problem but I'm willing to bet chkdisk was part of it. The PC seems fine now 7 hours and no problems yet.

 

http://vlaurie.com/computers2/Articles/chkdsk.htm

The article was written for XP, but the focus and concepts are the same for Vista.