Did you look at the "Can't Sync" thread right below yours? Here's a link:
http://www.animemusicvideos.org/phpBB/v ... hp?t=76047
It's probably a preview issue for you, too. Copy and pasted from that thread, see if it applies:
"Don't try to rewatch the entire video in WMM - just preview parts as you edit. To view the entire video, export and use windows media player."
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If it's only getting out of sync after a while (by the time you get to the newly edited clips), it's probably a memory issue. Don't watch from the beginning. Click the mouse over the timeline right before the newly edited clips and only preview the new stuff.
If it's still not synced, it's a codec issue. That FAQ Black_Pearl linked to definitely covers codec issues. The basics - don't edit with divx/xvid/mpg/wmv or anything besides huffyuv and lagarith.