I am not running XP at the moment but I & others I know have sometimes let the hard drives get much fuller than recommended. I think probably 20-25% space would be recommended and defragging IIRC is impossible under about 15% free space. Not sure I recall any warnings with download prompts for downloads and you may need to be wary of that incase you are downloading malware. This is not really a Firefox specific problem. Some tips *Can you offload any large files and folders at least temporarily somewhere elsewhere maybe to CDs, Flash Drives * do you normally hibernate, stop doing that it frees up the hibernate file, which is the size of you RAM * IIRC XP as standard has cleanup options you may use see ** Description of the Disk Cleanup Tool in Windows XP<br/>http://support.microsoft.com/kb/310312