Also, pre-built PC's like dell, HP & other tend to skimp on chips, so it's possible that the mobo doesn't support the faster speeds like UDMA5/6 (thats 100/133MB/s).
The seagate 120GB should support UDMA5 (100MB/s) instead of UDMA4 (66MB/s).
The other thing to note is also that many DVD's are now coming out with copy-protections, and the computer has to get around these, as well as decrypt the actual movie.
It is common for these to take upto 1hr to decrypt & rip the DVD to the HD.
Worse, since most DVD's these days are larger than 4.5GB (usually 7-9GB's), the video has to be compressed to squeeze it onto a single layer disk. This can also take a long time.