There are a few things you can do.
1) Install TOP and see what is taking up the Processor time. (top is a command line tool in the resource kit)
or
2) You can install PULIST and do the same. (pulist is a command line tool in the resource kit)
or
3) Use the task manager to see what is taking up the CPU
or and this I would do after all others if no obvious answer jumps out at you.
4) Use System Monitor under admin tools to monitor the following:
Processor - % Processor Time
System - Processor Queue Length
Memory - Pages/sec
Memory - Available Mbytes
Physical Disk - Avg. Disk sec/Transfer
Physical Disk - Disk Queue Length
If Pages/Sec average is above 20 indicates that the computer requires more memory. This could also mean that you have bad memory stick as well.
If the Avg. Disk sec/Transfer counter is exceeds 0.3. The hard disk must be upgraded or replaced in order to improve performance.
If the disk queue length is higher than 2 this indicates that the disk is a bottleneck.
If you notice that the %Processor Time Counter is at 100% fore a prolonged time. This indicates that the CPU is the bottleneck, if the Pages/Sec counter has a low value. If that happens you should see 1, 2 and 3 and find out what is taking up the cpu time.
Ack, I forgot the high numbers for the rest of the itmes. Sorry ,you might have to look that up if you don't find your answer using the above info.