No need to delete your cache file when you can simply install the files and not worry about them being auto deleted after 30 days of non use.
http://www.thunder65.com/ut2004/Cachemi.rar
I've used Cachemi since the 2nd month UT2004 was released :D That url is my site (so much easier hosting it than trying to google it over and over).
If you can't open .rar (winrar) files, let me know and I'll upload a .zip (winzip) version.
Step 1 Download
Step 2 Extract (double click)
Step 3 Navigate to where it installed and double click cachemi (or ut2004mi)
Step 4 Click the configure tab (far right)
Step 5 fill in the path to where your ut2004 is installed (default C:\UT2004).
Step 6 click the save button
Step 7 Click on the Cache tab (middle)
Step 8 Highlight all files
Step 9 click the option titled "install only"
* Step 10 click the install button
* = Sometimes it won't install all the files you highlighted, simply highlight the ones that didn't install and click the install button. Repeat until nothing is left.
Step 11 close cachemi
Step 12 play :D
Not sure what your problem is (lot's of possibilities) but cleaning the cache will improve performance

If you lost your cd and have the old drive and have not reinstalled UT2004 or reformatted the drive, you can find your old cd key in the registry here ...
My Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Unreal Technologies\Installed Apps\UT2004