I would say that the most obvious cause of your problems would be lack of disc space!
Capture of DV footage as an .AVI file on a PC will eat up around 14Gb of space for every 1 hour captured

. So you say you have an 80Gb HDD, but the chances are that is the actual size rather than free space with Windows and all the other progs on the PC taking up a fair bit of the 80Gb. As the drive get full the files get more fragmented as the system searches for space which then reduces the performance of the HDD and causes more problems. I would recommend you get a second HDD and only use it for captured video - don't put any programs on it!
You might get away with the one HDD if you defragment it before captureing any video, but a new HDD will make life much easier, especially if you want to create a DVD as you need more space for the editing files and then file conversion. I always try to have double the space used for the origional capture when doing any editing.
Is the firewire card in the PCI slot next to the graphics AGP slot? If so then you should try to move it to another PCI slot as the AGP slot and the closest PCI slot on many MoBos share resources that can cause problems. The card being a generic OHCI ieee1394 interface card is not going to be the cause of any problems as this is what many people use. Even an expensive firewire card will be ieee1394 complient as this is the standards used for firewire.
Mark.