I came across the file tvdebug.log and found from Google that it's one of those miscellaneous files created by the ZoneAlarm firewall I'm using. Apparently it's a file that just grows and grows. I also found from Google that there's a 30KB freeware program called dellater.exe that I could use to delete it, rather like CCleaner does, on the next boot. ZoneAlarm will create the log file again, but it will now start off from zero bytes, not from a few megabytes.
But this means I have to run both CCleaner and dellater when I shutdown my PC. It would make life easier if CCleaner also dealt with tvdebug.log. Can this be added sometime? It already deals with some ZoneAlarm log files, but not this one.