Erratic filesizes in XviD

Okay, so each week I was encoding a show to 175MB using AVISynth>VirtualDub and compressing in XviD. One day it was absolutely fine, and the next day - without changing any settings - it started churning out inconsistent and erratic filesizes. For instance, an episode of a show with a length of 20:43 would give a filesize of 223MB, still using 178200 and my regular audio settings to get to 175MB (which worked the previous day). After a bit of messing around to try and fixing it but to no avail, I upgraded my XviD to the latest version. The same file then turned out files of 198MB instead of 223MB. Keep in mind this is even if I encode the file twice - the sizes would still be 223MB and 198MB respectively. For a few weeks I suffered with encoding the file about 8 times until I managed to find the number that got me 175MB, but this was becoming a serious issue and I didn't want to damage my laptop. I've tried soft-formatting and system restore, but the problem sticks. I have emailed tech support to try and find out how to hard format on a laptop with no floppy drive or obvious way to get to a command prompt, but haven't had a reply yet (it has been a week). I am wondering if anybody knows what the problem is, and if possible, can you suggest how to fix it? What puzzles me is how I didn't change any settings from one day to the next, but the problem arose. :( As a result, I'm not seriously behind in encoding. :/ Thanks to ANYONE that can help... you'll be a life saver.

is the 178200 your target filesize that you put in the second pass of xvid? if so then that perfectly fits what your getting in terms of filesize as 178200 + 128kbps mp3 == roughly 197-198. Your target filesize in xvid should be around 152000 if I remember correctly(thinking rather roughly about it).

it's even a sticky...