Just purchased it about 2 months ago so that should be close to the latest version.
Well, first off, before we spend any time troubleshooting it:
Try the latest version.
JRiver releases new versions here on the forum every 1-2 weeks (and beta builds 3-4 times per week, usually), so if you haven't updated in two months, then you're probably 6-10 public versions behind (possibly more depending on which version you downloaded originally). Updating isn't hard. You can just download the installer and "install over top" of the existing one (or use the updater built into MC under the Help Menu, set to
Latest). It preserves all settings.
Official XP support
was discontinued with MC21. They are not making any changes intentionally to break compatibility with it, or anything, and it is still expected to "mostly, if not entirely, work", but they aren't testing for it internally at all. Also, some of the video playback components (third-party ones, and LAV itself) are moving to using Windows 7 and newer APIs. You may have trouble with Red October HQ mode due to these changes, though I don't know if anyone is sure of the current status.
But, before we go down that road at all, try the latest build. I know Hendrik has been chasing some nvidia-related issues in Theater View, and who knows what else has changed in the interim. If that doesn't help, review the entire Troubleshooting Guide I linked to above as well. The most common cause of dead-locks in Theater View are GPU drivers or general problems in the Direct3D subsystem in Windows.