I've just had to faff about re-partitioning my hard drive because I'd run out of space on one of my partitions!
And the party to blame (and thank?) for 140gb partition being full is those folks over at Steam!
My I: Drive, affectionately called FAT DISK contains only one folder. One single folder: Steam
However! Using a rather neat piece of software, I've given Steam more space to eventually move into - Fantastic! It now has 293gb (About a 3rd of my 2nd drive) all to itself to enjoy! I personally see this as a bit of an omen. And what is that omen? To buy more Steam games of course!
If anyone is in a similar position (Having to delete games in order to install new ones) I suggest the partition adjuster I used pretty highly! It's called MiniTool Partition Wizard! Uses a graphical interface, just shift the boundaries as you please. Keep all your games and use up waste space!
(However, make sure you follow what it says, and make backups. Don't want to goose your PC do you)