Can I install software on HP iPAQ GPS System Pocket PC - RX5915 travel companion?

Yes you can install software like any other Pocket PC - Its a Windows Mobile 5 device.

Also of note - it does come with 2gig of storage space but most of that is taken up by the pre-installed GPS maps.

You'll have access to about 300 meg - which is still a LOT more than most Pocket PCs.