the pctv 300i is a semi-hybrid card.
it has an actual dvb-t tuner portion, great for when sentech pull finger and start broadcasting in earnest, but (and importantly) it has a "software" analog tuner.
now, software tuners dont work with mce, unless the installer has added a software mpeg2 encoder (these things do exist nowdays)
so, i reckon your card worked just fine with mce, because someone had added a software mpeg2 encoder package. and now, in vista, the drivers that ship with vista/windowsupdate only expose the dvb-t portion, and i would guess it doesnt give you many options when setting up tv...it just assumes you are using dvb-t and goes straight for a scan and gets nothing.
do a google for software mpeg2 encoder. and check if it works with vista. like this
http://www.google.co.uk/search?hl=en&safe=off&client=firefox-a&channel=s&rls=org.mozilla%3Aen-GB%3Aofficial&hs=apT&q=software+mpeg2+encoder+mce&btnG=Search&meta=
which should send you here
http://www.xpmediacentre.com.au/community/xp-media-center-2005-software/10594-get-most-cards-work-mce.html
(p.s.my kid has a 300i in his mce2005 box, so i know how it works, but we only use the dvb-t side)