Ok, a couple things could be going on:
1) Someone mentioned defragging your drive, this is a VERY good idea... Windows barfs on too may fragments, it seems to like continous free space only.
2) Our most excellent hardware member, cak, has a partition on his drive set aside for JUST the windows cache file.
3) Close down all other running programs. Popular programs with KNOWN virtual memory black holes include ICQ, Checkit Net Optimizer, Neoplanet and Windows! (lol, you can't close the last, but it certainly has problems!)
4) Try increasing your Unreal memory cache size. 64 to 128Mb should do it. The more space you can set aside for Unreal's own cache, the less it will have to ask windows for cache. (NOTE: This is not the video cache! The OTHER cache line...)
Hope this helps!