I also updated to Avast Internet Suite, 7.0.1466 & am experiencing the same problem on two different systems, one with Outlook 2010 64-bit & the other with Outlook 2003 32-bit.
I ran ScanPST to repair the Outlook files on both Outlook 2010 & Outlook 2003. I assumed that the changing Subject line was related to corrupt linkages within the PST. The PST files (both POP3 & IMAP) has a large number of errors.
Once repairs were completed, I restarted the systems, let Outlook run awhile, exited Outlook, & ran ScanPST again. The PST files once again had errors.
It appears that the Avast AntiSpam plugin is corrupting the PST files.
I'm also a paid licensee.