first if the .pst is greater than 2 GB then you have to use the mailbox cropping tool that you can get from microsoft.
if not, there is a trick I found a while back for fixing .pst files.
First back it up.
then, using a hex editor (any shareware/freeware will work) clear out positions 7 through 13 (just put a space there)...so that in essence you are cleaning out the following positions:
00007
00008
00009
0000a
0000b
0000c
0000d
0000e
0000f
00010
00011
00012
00013
Save it and run the inbox repair tool. This technique will also recover deleted items if the .pst hasn't been compacted. Works wonders if someone is terminated and they delete their mail thinking that it will be gone.