"will it be easy to either dual install XP/Win7" - yes, shouldn't be a problem, but the hard drive would need to be divided into at least 2 partitions, with the XP install going on first (preferably).
"Does Win7 have a compatibility mode that might run the mapping prog?" - I doubt the Win7 (being Starter edition) that usually comes with most netbooks will have a compatibility/XP Mode. But anything better (Home Premium & better) can be made to run XP Mode (basically a virtual XP PC within Win7) with the help of the free VMLite click here. Not sure how a netbook will cope with XP mde though, so the dual-booting option may be better.
"If I formatted the HD to install XP, how would the USB drivers be installed to enable use of a usb dvd drive?" - should be automatically taken care of by XP, but if not, a USB flash drive can be used to do this. If you don't have one, they are available starting at under £5 click here. Also check on Amazon. G