Hi Mat,

Dont forget that system updates will dump lots of crap onto your system. Also you might be running system restore and it might use some up.
Then the rest is either temp files (look on the net for info on how to check what is ok to delete) or windows bloat - the more they have to play with the less they optimise.

Like mike says with the .jpg files you can save as progressive and standard? and these can affect what can be used/read in various programs.

Si