Guys I have had it with vegas I can't figure this out...

I'm trying to render as a wmv file or mpeg but for some projects it just wont sync the audio and video right, like for one of my projects the preview is perfectly fine as far as syncing goes, but when I render it always goes out of sync I've tried many different kbps and settings still no luck :( ... what is the problem ?

I would really appreciate your help, I would like to render into wmv streight from vegas

my PC's spec:
XP Pro
Sony Vegas 6
2GB DDR Ram
AMD X2 4400
SATA Drives

Thanks