Network adapter becomes disabled and will not enable

The user has an XP desktop system that has begun to disconnect from the network now and then, with no obvious pattern as to when it happens.  When it does happen, the network adapter shows that it is Disabled.  If I click on Enable, it shows that it is trying to do so for a second or two, then says "Connection failed!".  If I reboot the PC, the adapter starts working just fine, like normal--for awhile, and then it eventually fails again.

So before I go buy them a new network card, I need to see if there are any better ideas on what might be happening other than a bad network adapter.  TIA