If it appears that your Exchange server is running completely slow but your processor and memory are not being utilized then run perfmon. It is more than likely related to a disk bottleneck. To resolve disk bottlenecks you should make sure of the following:
1. Log files are on a separate disk/array than your database file. The log files perform constant writes to the disk as to where the database file performs constant reads. If you can separate these you can increase your disk performance.
2. Optimally you will want to separate your disks into 3 arrays. One for the OS, one for the log files, and the last one for the database files.