You should have at least 10 % empty space on the hard drive, no matter what size, for it to function correctly, especially for the boot drive. 15 % would be the recommendation.
2 GB can easily fill with caches over time - everything your computer does, typing, browsing, chatting, addressbook, photoshop - anything you do that stays for a while in its memory.
Clean the hard drive to have those 15 % free space - 9 GB if it's a 60 GB HD and so on. If you can't do that, get a bigger hard drive.
With that little empty space, even the maintenance scripts can't run, so the Mac will become more and more sluggish and need reboots to function.
Also get rid of Azereus and other junk applications, and run some proper maintenance to clean additional caches and run daily, monthly and weekly, e.g. with OnyX
http://www.titanium.free.fr/pgs/english.html