Store.exe too large

I have an NT4 Server SP4 on a Compaq Proliant machine with Exchange 5.5. Spec 386mb Ram 9Gb SCSI HD(3Gb free) P3 450 25 clients. I noticed that the store.exe file gets very large and a virtual memory error appears, reboot then OK for a while. I serviced packed exchange to SP4 and compacted the priv.edb file from the command line.Ran optimisation which moved *.edb to larger drive and diskeeper, yet no difference. I still need to reboot too frequently. Is there anthing else I can do?