It's on page 888 of the Administrator's Guide for BEWS 12:To protect the Workstation versions of the supported Windows platforms, you must install the Remote Agent on each platform; however a license is not required for computers running Windows work...
We have experienced a similar issue on two Windows XP PCs out of roughly 120 (different hardware across all in general, the problem PCs are also different from each other). Nothing we have tried has been a permanent fix for getting the agent to work...