Vista x64 & Video Playback

Hello all, I've been using Vista Ulimate x64 version for a while now and well I finally have a complaint. Before I go into detail on my problem, I'll detail my current system:

CPU: Athlon 64 3200+
Mobo: Nforce 4
Memory: 3gb
Primary HDD: Raptor 36.7gb
Sound Card: SB Audigy 2 ZS
G-card: Evga 6800gt

That should be enough, if not let me know. Anyways back when I had XP I could play 720p videos without any audio desyncing at all! I couldn't play 1080p videos but I wasn't really into 1080p videos at the time. Well with a lot of my favorite shows being brodcast in 720p HD and finding apple's movie trailer section where pratically all trailers are in 720p and 1080p, I've tried to get my PC to play 720p videos. This just doesn't happen though. I have the latest custom sound driver pack (One by the famous Daniel_K), I have the latest graphics drivers, I also have the latest Windows updates and I am running the vista codec pack with the x64 component. I also have the x64 version media player classic.

I've tried everything and it seems as if I can't get a 720p video to play without the audio desyncing. If I play a standard version of my TV shows vs the 720p version, the standard version plays fine with virtually no audio desyncing. Is this problem just because my computer is old and Vista demands more or is it something else. Also, I've done a fresh restart, and tried 720p videos on both x86 and x64 media players, and I have nothing running in the background (beyond the norm like AV) like Firefox or IE or scan of any kind. I think my computer is just getting old, but I thought I'd check here first. Also I assume a better processor would be more beneficial in this issue.

Maybe there is a misunderstanding here. I don't think it's the fact that neither my g-card or monitor (which is a 19" lcd by the way) is HDCP that's my problem. Like I said back when I used XP I could play videos with a resolution of 1280x720p just fine, but since I've upgraded to Vista Ultimate x64, they don't play so fine anymore. I get back audio desyncing. I'm just trying to determine if it's aging hardware or something misconfigured.