I've no experience with PCTVDELUXE USBTV TUNER; however, with that said, I googled for it and had google translate the pages I found. If I'm reading correctly this card only captures in mpeg2 or mpeg1. Screenblast will open these files.

You have to be careful when editing mpeg files. These files are already compressed and the compression is lossy. Meaning that some details have already been lost. This is normally ok because your eyes can compensate for this. However, if you edit an mpeg file and then save it as an mpeg file then you are re-compressing and so will lose even more detail. You could save as an avi and then use some other encoding tool to re-encode but your best bet would be to see if the card will capture directly to avi.

If you just want to use Screenblast to edit out commercials, then I'd recommend a free program called Virtualdub. You can cut out just parts of the file and it will not re-encode.
You can get it here: Well the site is down right now or I'd give you a specific link. Just go to www.dvdrhelp.com and along the left edge look for Tools. Pick that link and then put vituraldub-mpeg in the search field and you should find it.

Hope this helps.