Use PNG format for screenshots. No compression artifacts, and smaller filesize.

(Of course, saving the JPEG image, with the artifacts, to PNG will increase the size, as it preserves the defects.)
Psh, only a 100GB partition.
At least you realized it now. I remember seeing a 5 year old PC that had never known the love of a good defragmenting program. Norton Speed Disk's drive map (back when Norton Utilities were better than they are now) was an utter mess. Wee little pieces of data all over the place.