It's extremely likely your problem is
nVidia's drivers.
There's an issue with A64s + SP2's DEP + UT engine games + nVidia's crappy drivers (yes i said that).
Disabling DEP may help fix that issue, since nVidia doesn't care to.
Here's a copy & paste of me explaining the drivers fix to someone:
Ah, the joys of Nvidia's drivers...okay here's what is going wrong.
With SP2 came the wondrous DEP, & unfortunately, the newer versions of Nvidia's drivers have major issues with DEP & A64s & UT2k4...& then you get those nice BSODs (Blue Screen of Death).
There are a couple ways to work around this.
You can either disable DEP entirely, or use older Nvidia drivers.
For some reason, if i understand right, disabling DEP just for UT2k4 does not always work.
However, i would still try it first.
Go to My Computer > Properties > Advanced tab > settings button under Performance > Data Execution Prevention tab > select "Turn on DEP for all programs & services except those I select" > now add UT2004.exe (from your UT2004 folder) to the list.
Now that may not work.
If it doesn't, you can turn off DEP for everything.
Go to My Computer > Properties > Advanced tab > settings button under Startup & Recovery > select Edit, & you will be in your boot.ini file.
It should look similar to this:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
On that bottom line, remove the noexecute & replace it with execute
Make sure you save.