slow performance/ high CPU usage system process

for several weeks now I've been experiencing slow performance of our
file-server at rondom times. When people are trying to open a folder or a
file, or save a file, it sometimes takes up to 20 seconds before the system
responds.

An analyse of de server and it's services shows me that the System Process
(NOT Idle) is constantly consuming about 30% of the CPU.
Analyzing the System Process shows that the driver SRV.SYS is responsible
for it.

Details of the server:
HP DL380 G3
1 CPU, Intel Xeon 3.06 Ghz
1 GB Memory (less then 450MB in use)
HP 5i Smart Array (on-board) for OS-discs (mirrored)
HP 642 Smart Array for Data discs
LSI ultra320 SCSI adapter 2000 series for LTO-2 drive
Data-discs are in a Storage Array (MSA30), 6 x 72 GB RAID-5
Backup-software = Veritas BE 10

The problem is that I am unable to solve this problem and that I'm out of
options. Can someone please help me.

Thanks