I've used a usb hd tuner on my Eee netbook and it works fine so i'm pretty sure a pentium 4 can handle a hd tuner card.
As an example of the requirements (from the usb hd tuner i used):
Windows XP/ XP MCE/ Vista TM / Vista TM MCE
CPU: Intel Pentium 4 2.6GHz with HT (Hyper Threading) or Equivalent AMD Processor
Intel Pentium Mobile 1.8GHz
512MB RAM or above
VGA card supports DirectX 9.0c or above
Sound Card