There is a bug in the Windows XP Service Pack 3 update that decreases the data throughput (speed) of the Firewire interface from 400 Mb/s (S400) to 100 Mb/s (S100) that can significantly affect the performance of your Firewire based SDR. There is a Windows XP "hotfix" When you upgrade to Windows XP SP3, an installed 1394b FireWire device reverts from S400 speed to S100 (http://support.microsoft.com/?kbid=955408) that resolves this issue.
Click on the link above to access the aforementioned hotfix. Follow the instructions to download and install the hotfix
Event though the hotfix explanation indicates that the hotfix is only applicable to 1394b devices, it is applicable to 1394a devices like the FLEX-5000 and FLEX-3000 and is highly recommended that you apply this hotfix.