Problem wth System hardisk space

CASE:-

My 120 gig C drive partition yesterday reported 62.4 GB free. Today I get a warning telling me free space is dangerously low.
Checking it I see 53 mb free and checking it again 5 minutes later I see 26.7 mb free.System Restore is configured for 11% (13.21 GB). The other partition on this 250 gig drive is consistently reporting 69.4 GB free. 
When I do a restart I get totally different numbers for that C partition such a 43 GB free and something else again an hour later. 

Follow the steps in the following methods

Method 1 : 

Try running Disk clean up and check 

If you want to reduce the number of unnecessary files on your hard disk to free up disk space and help your computer run faster, use Disk Cleanup. It removes temporary files, empties the Recycle Bin, and removes a variety of system files and other items that you no longer need.

1. Open Disk Cleanup by clicking the Start button  , clicking All Programs , clicking Accessories , clicking System Tools , and then clicking Disk Cleanup .
2. In the Disk Cleanup Options dialog box, choose whether you want to clean up your own files only or all of the files on the computer.   If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3. If the Disk Cleanup: Drive Selection dialog box appears, select the hard disk drive that you want to clean up, and then click OK .
4. Click the Disk Cleanup tab, and then select the check boxes for the files you want to delete.
5. When you finish selecting the files you want to delete, click OK , and then click Delete files to confirm the operation. Disk Cleanup proceeds to remove all unnecessary files from your computer.

The More Options tab is available when you choose to clean files from all users on the computer. This tab includes two additional ways to free even more disk space:

Method 2 : 

Improve performance by defragmenting your hard disk 

Fragmentation makes your hard disk do extra work that can slow down your computer. Disk Defragmenter rearranges fragmented data so your hard disk can work more efficiently. Disk Defragmenter runs on a schedule, but you can also defragment your hard disk manually.

1.    Open Disk Defragmenter by clicking the Start button  , clicking All Programs , clicking Accessories , clicking System Tools , and then clicking Disk Defragmenter .   If you are prompted for an administrator password or confirmation, type the password or provide confirmation.

2.    Click Defragment Now .

Disk Defragmenter might take from several minutes to a few hours to finish, depending on the size and degree of fragmentation of your hard disk. You can still use your computer during the defragmentation process.

Method 3 :

Perform check disk on your computer.

1.    Click Start, click Run, type cmd, and then click OK. A Command Prompt window opens. 
2.    Type chkdsk /r, and then press ENTER.

You are prompted to schedule Chkdsk to run when the computer restarts. 
3.    Press Y, and then press ENTER. 
4.    Restart your computer. During startup, Windows will scan your disk for errors, and Windows will try to recover any files that are located on damaged areas on the disk.

you have some space you should install ncdu - ncurses disk usage
>> viewer. It shows you where all the space has gone and enables you to
>> navigate trough the directories. You can even delete files/dirs with it.