You can accomplish this with Tweak UI, part of Microsoft's PowerToys for Windows XP.
Download and install Tweak UI (or click here for the 64-bit version), and fire it up. Expand the General section, and click on Focus. Check the "Prevent applications from stealing focus" checkbox, and then click on "Flash taskbar button X times" (where you can set the number of times it flashes).
The default value is 3 - or at least on my computer it is :)
Here's a picture to show you exactly what I mean:
Start > Run > type REGEDIT and click OK.
Navigate to the following key:
Create or modify the DWORD value ForegroundFlashCount and set the value to 3.