Vista: extreme slowness when working with large files

I just downloaded my Civilization 4 game (purchased from Direct2Drive),
which is 1.3 GB in size. It took Vista 10+ minutes to get to the point
where it brought up the UAC warning, during which time the Windows Explorer
window was completely frozen. This would have taken 30 seconds tops on XP.
Is anyone else having issues working with large files?

Here is my system configuration:
Opteron 165 dual core (clocked @2.25 GHz)
2 GB PC3200
2x120 GB 7200 RPM HDD (Raid 0); 250 GB external HDD
512 MB x1950 Pro PCIE video card
SB Live 24 bit
939 Dual SATA 2 mobo (Uli M1695 chipset)

I ran HDTach, and got a Burst speed of 142.0 MB/s, with average read of 62.2
MB/s, which is comparable to what I got in XP, and also comparable to these
review benchmarks of this board.
http://www.ocworkbench.com/2005/asrock/reviews/939Dual-SATA2/usb-diskperf1.htm
Therefore, it seems highly unlikely to be a motherboard/driver issue if
HDTach performance is fine.

Does anyone have any suggestions for speeding up file access in Vista? Is
this a known issue?

Thanks.