Unused HD space suddenly missing.

Hi, 1st timer here, my problem is that last night i checked my HD space and it showed i had 3.47GB of free space left. About 30mins later while i was watching a movie, windows popped a message saying i was critically low on HD space, I checked again and it suddenly showed I only had 100MB left.

This is the 2nd (3rd?) time it happened to me, the last time was about 3-4weeks ago and I did a scandisk, defragged, and used CCleaner but nothing changed, so this time I'm looking for some help before I do anything that might compromise my PC even more.

I think about 1 1/2 - 2months ago this happened for the 1st time, but i cant be too sure bout it, on all 3 times i lost bout 3GB++ of space after a few hours or after i went to sleep.

My PC specs
XP Pro SP2
Gigabyte GA-8IPE1000-G Motherboard
250GB HD partitioned into 100GB and 150GB(1GB free space)
1GB DDR1 RAM

Last formated my PC about 6months ago.
Also, I exchange a lot of movies and anime videos with my friends so my HD space gets used a lot and i defrag about every 3weeks.