[Imp]

I think what DRG is implying is you have a few options
1) You can prove you bought the original game either by taking a screen shot on the screen that lists the version number if they will accept that
or
by allowing remote access & letting them look at your computer

Then despite the fact they dont need to they will probably be nice & offer you a CD or electronic download at a discount.

2) Pretty sure if you have installed patched in order & it works fine there is nothing wrong with your install, The problem is with user generated files as in game saves both normal & PBEM player generated maps OOB changes etc not when a program is actually being installed

So what you can do is copy the entire game folder & move it to a folder called say games & use that.
Also back it up elsewhere as in on another drive or CD naming the folder MBT & patch number. Now if you ever need to reinstall just copy it & apply any patches AFTER those already installed.
No need to repurchase