In addition to user notification - there is a tab in agent configuration section where you can turn off mail scanning. Give a try!
In order to ensure that all computers pick up policies from ePO, restart the computer after HDLP deployment as soon as possible.
Disable the "Show progress bar when outgoing emails are processed by the DLP Agent" option under Agent Configuration -> Email probe. I have seen this pop up come up even if HDLP is not the culprit. e.g. Network issues.
I do not believe XP Tablet Edition as an operating system is supported. Please check with McAfee support.
You would want to spend more time with the Proof of Concept / Test Environment before beginning a Production Deployment.
Most of the issues can be avoided that way.