Not just the temp folder .... sounds like the System drive is full.
Windows "likes" 20-25% of the HDD as free space
It's ridiculous ... but when you download a file ... you download it to your temp folder and then windows copies the temp file to the folder you chose.... this requires about 2.5x the size of the file
a dvd = 4.3Gb therefore you need a minimum of 10Gb to download it
the page file = 2.5x your RAM so 1gb of ram needs 2.5Gb of disc space to support it
your waste bin will "take" up to 10% of your drive (another 10Gb?)
all in all it's amazing anyone ever gets a machine to load!!!
clean out your temp files
(right click a drive in my computer | properties | disk cleanup)
while you are in there defag the disc
Look in the windows folder ... delete any files that are blue and called something like
$NtUninstallKB921503$
but not $hf_mig$ (you need this)
Then think about buying a second HDD