Vista - Update = Hard Drive Space?

For the longest time, Vista seems to have been reporting my hard drive usage wrong. Most recently, it said around 23GB free (of a 200GB hard drive, 178GB after recovery partition and other). When doing a manual count I get nowhere near that. Just from programs, personal files, and system files I can find I get a discrepancy between actual and reported of about 30GB.
Then when I just ran Windows Update (17 new important updates today), the free space jumped to 47GB.
This obviously isn't a problem now (although over time the usage it seems to creep back up to where it was), but does anyone know why this happens? Is it really just cleaning up 20GB-worth of fragmented or old files as part of creating a System Restore point or something?

Whatever you were adding up gives you the wrong result because you are not "seeing" all the used space. I suspect the difference is the shadowstorage. I made a little write-up about it because the question comes up all the time:

With each install and uninstall, Vista writes a shadow (restore point) of about 1GB - plus one is written each day. For that purpose Vista reserves 15% of your OS disk partition and allocates/uses it as needed.
Find Command Prompt (in All Programs > Accessories > Command Prompt), right click on it and Open as Administrator. Into the little black window type VSSADMIN LIST SHADOWSTORAGE and hit ENTER. It will show 3 numbers:
Allocated - that is the amount that it has grabbed from your OS partition at this time
Used - this is the amount currently used
Maximum - this is the ultimate amount it will allocate and use
Once you reach "maximum", it will reuse the space deleting the oldest shadows for the storage of the newest shadows. With e.g. an appr. 200Gb of OS partition you should expect a maximum of 30GBs that are reserved for the system and that you cannot use. If your OS partition is larger or smaller, the shadowstorage will be accordingly (always 15% max.) But the restore points (shadows) are required the day you need to do a system restore.

PS: The reason why you suddenly have more space is that the recent updates may have wiped out your shadows. Check that with VSSADMIN LIST SHADOWSTORAGE

wow, that looks like that's exactly it. almost 27gb reserved for shadowstorage. i never realized that, thanks!

You are welcome.