Can't read disk in USB enclosure (Vista, XP)

I'm presently abroad, away from my main computer, and relegated to a laptop.
Before I left home I backed up my data onto a WD 750 GB disk. I'm not sure
what format it's in - whatever XP did by default. I planned to simply
enclose the drive in a USB case upon arrival and access my data that way.

Unfortunately, although I've tried 2 USB enclosures I am unable to see the
disk in either XP or Vista. The USB device appears in the Task Tray, and
correctly identifies the disk, however it doesn't appear in Explorer.

When I look under System Administration > Storage, the disk appears as
"Foreign" and asks if I'd like to convert it to a Basic disk.

If I do this however, won't I lose my data? And while we're at it, where is
my data, and can anyone think of a reason why I wouldn't be able to see it?

Thanks,

Tom