I found this in a few different places around the 'net - hopefully it's helpful:

Hard Disk Performance Is Slower Than You Expect

Symptoms
After you install Microsoft Windows XP, hard disk performance may be slower than you expect.
Note: Hard disk performance may be even slower when your computer performs many small hard disk read/write operations.

Cause
This behavior may occur in the following situation:

You use Small Computer System Interface (SCSI) hard disks in the computer.

-and-

The hard disks are formatted as NTFS. Resolution
A supported fix is now available from Microsoft, but it is only intended to correct the problem described in this article and should be applied only to systems experiencing this specific problem. This fix may receive additional testing at a later time, to further ensure product quality. Therefore, if you are not severely affected by this problem, Microsoft recommends that you wait for the next Windows XP service pack that contains this fix.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information about support costs, please go to the following address on the World Wide Web:

http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

Note: In special cases, charges that are normally incurred for support calls may be canceled, if a Microsoft Support Professional determines that a specific update will resolve your problem. Normal support costs will apply to additional support questions and issues that do not qualify for the specific update in question.