this appears to be a Windows problem.
Does the PC have a separate wireless card?
You could try a cheap PCI wireless card or even a USB wireless dongle.
I have an Airport Express set up to receive music from iTunes and play on my receiver. I have two machines linked to the AE, my laptop, and my desktop. My laptop never has an issue. It linked to the AE when first set up, and it "sees" it any time I would like it to.
However, all of my music is housed on my desktop. The desktop will see the AE and function fine with the AE when the PC is first booted up. Then, in anywhere between 15 minutes and a day, the AE will disappear from iTunes (grayed out) and the only way to fix this is to reboot the PC. As soon as the computer is restarted, iTunes sees the AE again just fine, until it "forgets" it again whenever it feels like.
When it is not seen by iTunes, the AE Utility on the PC also does not see any AE units (however, if I load AE Utility on my laptop, it sees it just fine)
Any thoughts on how I can look into fixing this?