I have a Dell with XP Media Center. The current TV tuner card doesn't understand my HD signal coming from my outside antenna. If I purchase a HD tuner card will the correct the problem or do I have to go through a converter box like I do my standard analog TVs?
HDTV is all done as digital signals. But ALL OTA transmitters, HD or not, went digital earlier this year, anyway. Most of those signals are in the old UHF band from channels 14 through 69, some still in the VHF Hi band channels 7 through 13.
HOWEVER, digital TV signals are done two different ways. On cable systems they use QAM signals, and for now those are not encrypted in any way so they are called "Clear QAM" digital TV (HD or not, same thing). On the other hand, OTA transmitters are using a signal system called ATSC. Lots of PC tuner cards these days can handle both types of digital TV signals. Yours just might be old enough not to be able to do both. But first look through its settings and see if you get to choose whether it is handling ATSC or QAM signals. Maybe all you need to do is set it to ATSC.
Also check the details of your tuner card input. Some used separate input connectors for OTA signals from an antennas, vs. a cable TV service input.
A set-top converter box such as the ones to adapt older TV's will tune in new OTA signals of the ATSC variety and convert them to an analog signal on channel 3 (or whichever), BUT I really doubt they will preserve the HD quality.
I use a Hauppauge WinTV-HVR-2250 in my HTPC and have been very happy with it. It has an internal splitter, so you just plug in a single cable, but can record two HD signals at the same time. The HVR-1800 linked above will only record one digital and one analog signal at a time.
shortstuff_mt raises a very good point. But watch out for the phrase "dual tuner". For example, as shortstuff_mt says, the tuner linked by shabaa actually has "dual tuners" for TV (plus an FM radio tuner with its own dedicated input connector). But they way they are set up, one uses one input connector and can ONLY handle analog TV signals which, by now, are available ONLY on cable TV. The other TV tuner with its own input connector can handle all types of digital TV signals, but NOT analog, and only one channel at a time, of course. So you could watch / record two channels at once as long as one is digital (from whatever source) and one is analog from cable.
The alternative tuner shortstuff_mt references arranges things differently. One of its two input connectors is dedicated to the FM tuner, just like the previous tuner card. But the other input connector feeds via an internal splitter into two identical TV tuners. Each of these can handle it all - NTSC analog TV (only from cable), Clear QAM digital TV from cable, and ATSC digital TV from OTA transmitters. So in your case if you hooked one of these to your OTA antenna as TV input, BOTH tuners could tune in different ATSC digital TV channels (HD or not) independently for viewing and / or recording. In your case where your TV signal source is OTA antenna which is ALL ATSC digital signals (unlike cable with some NTSC analog and some Clear QAM digital), this is the kind of "dual tuner" card you need.