Update to 2.5 ?

[Mobhack]

I presume you downloaded and installed (in order)
1) The base free game download (100 Mb)
2) the 2.0 upgrade patch (13 Mb)
3) the 2.5 upgrade patch (3.5 Mb)
from the downloads page at shrapnel
(+ optional)
the 2.51 upgrade patch for restoring turret orientation (a sticky thread at the top of this main forum page).

And have problems. I would say either
1) You installed one of the patches to the wrong path
or
2) You had a bad zip file download
or
3) You missed out one of the files (perhaps the main game download?)

First - check the size of the downloads you have saved to disk against thoose noted on the http://www.shrapnelgames.com/SPCamo/wSPMBT/6.htm page.
If one or more of these is wildly out - re-download it as it may be a bad/corrupt/incomplete download. (I think those are the sizes for the zip files and not the extracted installer exes).

Always extract the exe to disk from the zip and then run it - although windows will allow you to run an exe from inside an opened zip, I would think this not a very reliable procedure!.

I would then try running the 3 EXES again, in order, and see if the problem goes away (check the exact path the installer for each patch wants to use, especially if you have not used the default path).

The other possibility is a bad disk sector or bad disk structure or lost clusters etc, so it may well be worth running scandisk to at least remove any basic problems with the hard drive. If necessary, try a scan of disk sectors (Go watch a movie or do it as an overnight run if you do a sector scan as it will take a long time ! .

Cheers
Andy