Disclaimer!!
Some of the advice I'm about to give you has the potential to make recovery from computer issues extremely difficult, if not impossible.
Further, a couple things will disable features of your computer that you may want to use.
I take no responsibility for crashes/problems that occur later; follow at own risk.
With that out of the way, here's what's probably hogging your space:
I'm going to presume you've got a pretty large disk (>500gb) and a few gb of ram (4 or >). If any of that info's wrong, let me know.
If you're running windows 7, scratch 15gb right now for the os alone.
Compressing the windows folder can help with this, but you probably won't get back more than a couple gb.
If you're running windows xp, its 2-4 (maybe more, but no more than 10 if you have a whole lot of stuff installed) and again, compression may give you 20-30% back if you're lucky.
The items you're not seeing in c: are these:
*pagefile
*hiberfile (hibernation file)
*system restore
First, let's work on the pagefile. Before you do anything with this, make sure you have more than enough ram to keep your os happy on memory alone. This means:
*windows xp: 1gb or more, 2gb highly recommended
*windows7: at least 4gb
If you don't have this much, you're likely to see performance issues and possibly crashes if you disable or shrink the pagefile.
Pagefile is equal to your ram (if you have 4gb ram, 4gb disk space is used.)
You can change the pagefile size (or disable it entirely) by doing the following:
go to control panel>system
in windows 7, go to change settings
go to the advanced tab, and select the virtual memory button.
You'll have a list of all the drives on your computer, and what their pagefile status is.
To change this, select c:, tab over and set it to custom size.
Warning! if you disable the pagefile on all drives you'll get the most space, but you may cause your system to become unstable, and if it crashes, you won't have debugging information.
In custom size, set it to something like 32-512 mb (min-max).
That'll give you a sizable chunk of disk space back, but we're not done.
Next up is system restore. Exit out of the pagefile dialog, choosing ok until you're back at the system properties window. Do not reboot yet.
Make your way to the system protection (or system restore) tab.
The instructions for configuring system restore varry from os to os, so I'll cover them individually.
For xp:
If you'd prefer to disable system restore completely, simply check the "turn off system restore on all drives" box. This will however, prevent you from using system restore to recover later, so I'd advise against it unless you want to reformat every time there's a problem.
To change the amount of space used:
Tab over to the list of drives, and select c:
press tab, and select settings.
Now simply move the slider around to control what percent of your disk can be used by system restore.
If you have a large disk, its perfectly fine to set this to 1 or 2%.
Press ok, apply and get out of system properties. Do not reboot yet.
for windows 7:
Again, from the system protection tab, do the following:
Tab over to the list of drives, and select c:
tab over to configure, and select it.
You now have the option to either disable system restore (see above for warnings) or to move the slider to to change the percentage of disk used.
Again, if you have a big disk, 1-2% is perfectly fine.
Once you're done, exit out of system settings, answering no to reboot prompts.
This should give you an extremely large pile of disk space back (up to whatever percentage the slider was at). System restore can hog 50gb in very little time at all.
Finally, we're going to work on the hiberfile.
Hiberfile, like pagefile, is as big as your ram is.
If you don't use hibernation (or hybrid sleep on windows 7) you can go ahead and disable this, and reclaim some space.
To disable hibernation (any os):
press windows r to open the run dialog.
type cmd and hit enter
now type the following:
powercfg /hibernate off
exit
and reclaim another pile of disk space.
You'll need to reboot for these to take affect, so go ahead and do that, and enjoy a nice, free drive.
