Trouble reading DVD disc 2 in PC

Hi,

I recently rented X2: X-Men United, the widescreen version.

Problem 1:  Disc 1 (the movie) plays in my computer.  Disc 2 (the extras)
doesn't.  When I try to explore the disc my computer asks me to insert one
but the disc is already in the player.

Problem 2:  With Disc 1 in and the DVD menu up, I choose languages, then
English DTS.  The first time I chose this option the movie played with no
sound.  I swtiched it to 5.1 Dolby Digital and it worked fine.  The second
time I chose English DTS the software player hung and corrupted memory.  I
wound up rebooting the computer.

Can anyone help with these?  Thanks in advance.

My computer:
- SONY DRU-510 dvd+ burner
- DVD driver:  what came with WinXP
- NDVD 2.0 software player from Nvidia
- sound:  motherboard optical out connected to Pioneer receiver VSX-D912
which can decode DTS
- sound card:  none
- sound driver: Realtek AC97
- 3.0 ghz processor with 2gb of RAM
- Win XP Pro