Computer not reading DVD

Hi! I just spent a ton of money having my VHS tapes converted to DVD. I got the DVDs back, and they won't play on my brand-new Dell Dimension 5100. They also don't play on my old DVD player. However...they DO play on my new DVD player AND on my G4 Macintosh computer (which is about 5 years old). So I can't figure out what the problem is. My dad and my brother can both play the DVDs on THEIR Dell's... but their computers are several years old. Why would my NEW computer not be able to play the DVDs? I asked the guy who converted them what they were... he said the DVDs are DVD -R. He said they should play using Media Player or Power DVD. Both of those came with my new Dell, but neither will play the DVDs. I am new to the whole DVD thing, so I have no clue what to do next. My hope was to open the DVDs and edit them using ULead software (which I have not yet purchased). But since I can't even open them, I'm at a loss as to what to do next. Thank you!

Julie