The _RESTORE folders shouldn't occupy more than 12% of your C: drive or
400MB if the drive is <4GB.  If more than that it would appear that System
Restore (SR) is not discarding (FIFOing) older data correctly, possibly
because at some time the system was not immediately rebooted following a
state change (switching SR either off or on).

So, my first suggestion is to reset System Restore:
To reset System Restore:
System | Performance | File System | Troubleshooting and check "Disable
System Restore", Apply and IMMEDIATELY reboot.  This will flush you
restore folder and erase all checkpoints, then,
System | Performance | File System | Troubleshooting and uncheck "Disable
System Restore", Apply and again IMMEDIATELY reboot.  This should now
automatically create a new checkpoint immediately following the restart.
Finally adjust the space allocated to the restore folder as explained
earlier.

If this doesn't clear down the _restore archive (and it doesn't always
when users have experienced similar problems) the next suggestion is to
boot to DOS using a floppy and delete the entire _RESTORE folder structure
from there:
a) Boot to DOS using a floppy.  Do NOT choose "Minimal Boot" from the menu
when booting from a floppy but rather choose "Start computer with (or
without) CD-ROM support" otherwise the ATTRIB command will not be
available.
b) At the DOS A:\> prompt, type:

ATTRIB    -H   -S    -R    C:\_RESTORE
REN    C:\_RESTORE    OLDREST

c) Remove the floppy
d) Reboot your PC
e) Delete the folder C:\OLDREST
f) Check that an automatic system restore checkpoint was created.
g) Finally adjust the space allocated to the restore folder:

Finally you may want to give system restore the once over by carrying out
the following quick test.
a) Create a shortcut on your desktop to a file.
b) Create a manual checkpoint
c) Delete the shortcut
d) Restore you PC to the checkpoint you created.
Was the shortcut restored?  Did you see any error messages?