This is excerpted from an MS article for w2k resource kit. Most of the numbers should still be applicable to newer version of OSes.
Resource
Object/Counter
Threshold
Comments
Disk
PhysicalDisk\% Disk Time
90%
Disk
PhysicalDisk\ Disk Reads/sec, PhysicalDisk\Disk
Depends on manufacturer's specifications
Check the disk's specified transfer rate to verify that the logged rate doesn't exceed specifications.(1)