XP systems progressive failure first NIC, USB, CD

About 3-4 months ago, we had a system that began to run slow; then it stopped working with the NIC; followed by the USB stopped responding; and finally the CD drive failed to read. With each step the system becomes slower and slower, until at the end you can login and leave it loading personal settings for hours.  It usually will load to the desktop if left running overnight.   There are no error messages and the devices are listed under Device Manager. The NIC may or may not be displayed in the Network Connections.  All virus scans come back negative for Trojans, spyware or viruses.  All of the above happen only in Windows.  All tests of the physical hardware and scans done before booting Windows show no hardware errors.  This problem usually occurs suddenly, without warning and without any hardware or software changes precipitating it.

Now we have experienced10-12 more systems with the same problem. The machines are several models of Latitude notebooks and Dimension and Optiplex desktops.  The oldest system was a few years old, the youngest system less than a year old.

Dell support instructs us to reload the OS. Once XP is reloaded, the system is good to go (with the exception of one machine that has had the problem twice).  We have only seen the problems on the Dell systems, but that is not surprising since we are a mostly Dell shop.  So far the dozen systems has been an annoyance but with several hundred systems installed, if this continues we could be in serious trouble.  Has anyone else seen this problem?