Losing disk space rapidly,need help/advice

[Rhodes]

The free disk space of my c: drive is about 600/500 megas! It's low, I know. And get a bit lower when I'm seeing streaming videos, downloading etc.
But today, it drop to 30 megas without any explanation. I ran disk cleanup, the "disk free up" from TuneUp Utilities2008, deleted the files of the Temp and Temporary Internet Files directories manualy. Did the same to the other user profile of my Pc. Only got 150 mb of free space.
When this happend I was playing F-117 in dox-box, but I don't think that this is the cause!

After schearching around I found out a huge file log of 800 mb, the log file of Dr.Watson. Did a little recherch in the net and deleted the file, got 900 mb of free space.
But, since I think the sudden drop of free space is not related to the big log file (one thing got me to look closer and discover a big file that possibly had that size even when I had my "normal" free disk size) I asking all the guys here, if You have an explanation, an idea and an advice for this odd thing.