I don't have the answer yet but Win 7 clearly has a serious issue reading data from DVD drives.
Putting a data DVD in the drive, and then opening the drive in Win 7 Windows Explorer is taking an *excruciating* long time to complete. Tested the same drive with the same DVD-ROM in a Win XP computer and the response was pretty much immediate.
Putting a *CD*-rom in the drive results in normal quick performance. Changing the drive from DVD/RW etc to plain DVD-ROM didn't make much difference - at first I suspected Win 7 was trying to be clever with a writeable DVD drive but that wasn't it.
When you search around the web there are thousands of posts from users seeking help with this problem, but most threads go through the lines of "ensure your AV is turned off", "check your drive hasn't got errors and fallen back to an old/slow read mode (PIO)" "make sure you haven't got malware" but it's clear to me the issue is related to Win 7 software support for the DVD drive.
Win 7 seems to be doing loads of IO to the drive, maybe it's indexing the files, or it's trying to create a thumbnail of everything on it, but the throughput for your application (in this case I'm just browsing in Windows Explorer and eventually (many minutes later) clicking on a msoft installer) is excruciatingly slow.