Timestamps drift from real time

I've been using Wireshark/Ethereal for several years and now I observe a very odd behaviour (Wireshark 1.4.2/1.4.6 and PCap 4.1.2) on a HP Compaq 8000 Elite CMT with Windows XP SP3 32bit.

When I start the capture, the timestamp of the immediately captured packet is okay. The more time passes, the more does the timestamp deviate from the real time. That is, after 10 seconds of real time passed, Wireshark will timestamp that only 4 seconds passed - no matter what the timeview format is. The clocks completely differ after some time (the OS clock will show something like 11:42:15 and when I provoke a packet then, the timestamp might say 11:40:57 or in the Seconds since Capture view, the first packet has 0 seconds, then I wait 10 seconds, send a packet and the packet has just 4 seconds as timestamp).

I've other software that timestamps received packets and this other software works though - the timestamps are correct. I can run them simultaneously, too and see the difference between the timestamps for the very same packet, up to minutes, if I let Wireshark run long enough. (As far as I can tell, Wireshark/PCap seems to retrieve the current time once and then internally counts the time while the capture is running - and this does not work properly on this machine for whatever reason).

I've reinstalled Wireshark and PCap, deinstalled nearly every other software etc. I've checked the BIOS if there are time related settings... right now I'm at a loss what else I can do. Wireshark works fine on my other computers (be it XP or Linux).

Additional: This is not just a timestamp delay due to performance issues or network traffic. The timestamps are plain wrong. It happens with single packets, name resolutions disabled etc. I've two network cards in the machine and the behaviour is the same for both.