You’re not going to like this…delete files.
Movie, Video, and Music files are huge (ESPECIALLY movies!). Move the music to a thumb drive (aka: USB key/drive)
Delete the movies/TV programs/Music Videos that may or may not have been pirated…
Remove the bloatware that came with your computer
Clear your Temp Files
Delete games (etc) that you no longer play.
Run the Disk Clean-Up utility that came with your Windows program.
Defrag the HDD.
etc…

Oh, if you have documents you have to keep, but don’t need continuing access to, compress them with WinZip or a similar program (7-Zip[?] is free and as good as WinZip).

Oh…also, go to a the homepage of a reputable Computer Security company (Symantec, etc), and have them run a free check of your system to see if you have any "issues" taking up critically needed HDD space. (Presuming you’re either not using, or are using FREEWARE Security software, you’re probably infected 50 ways from yesterday – or, "Six ways from Sunday" as the old expression goes! )

Short of that…buy a new (larger) HDD and have it installed, and re-install all your games on *it*, leaving the smaller HDD for Windows and System files.

Good Luck!

Wrath of Lich King but.....?