available space

Win 98 here. When I check system information, I normally see over 1400mb of available free space. All of a sudden, I have less than 100 mb,and it caused the CD burning things I do to wig out. Trying to figure out what has filled up so much space? I did a Find All Files created in the last 5 days operation, and the biggest files there are system.dat (10197kb), and there are a bunch of Mozilla cache and other Mozilla files. I did click on ok to download a Firefox update that popped up yesterday,and there sure are a lot of Mozilla files in the Find Files. Any suggestions on clean up to restore available space appreciated.