New system locking up in Vista 32 bit

I recently built a new system from scratch. The specs:

Ultra Aluminus case w/ included 700w PSU
ASUS P5N-E SLi Motherboard
e6600 Core2Duo CPU @ 2.4ghz (with a Zalman 9500 CPU heatsink)
2GB Corsair Dual Channel DDR2 RAM @ 800mhz
500GB Seagate SATA-300 Barracuda HDD, 7200.10
Sony 18x DVD drive

In addition, I'll be putting a eVGA 8800 GTX in there. But it hasn't arrived yet, so I'm currently using a stinky little FX 5500. Not sure of the manufacturer, I just found it laying around in my miscellaneous hardware bins when I needed a PCI card. It's got 256MB of VRAM but it's probably slow as molasses VRAM.

Now, I put Vista Ultimate 32 bit on there in preparation for playing DirectX games like Crysis. The install was fairly painless, but then I started getting problems while just browsing around - my system would completely lock up. Fans and stuff would be spinning still, but mouse, keyboard, everything else went into full on lockup. First lockup was on a restart after installing updates. Next lockup was when I was trying to install some updates. After that, it just locks up after 10 or 15 minutes, browsing the internet or transferring stuff from my external drive. The lockups even freeze my G15 LCD Keyboard, as the clock freezes on whatever time the system did.

As you can imagine, this is a really troubling me as this is my dream system and represents lots and lots of work. I don't even really know where to begin. I might rollback to XP this weekend when I go visit my father, who has lots of XP licenses available. It's a shame though, because I really like what little of Vista I've experienced.

I've read some stuff that might suggest it's a hard drive issue. My 500GB internal is actually flagged as a Removable hardware by Vista, but I haven't tried "Stopping" it as if it were a normal USB device. Apparently others have had problems with the 7200.10 barracuda and Vista Ultimate.

Can anywhere here point me to solutions? I'd be forever in your debt.