I have an iPaq hw6915 that keeps crashing even though there is still 60-80% battery life in it. I've had it for about 2 years, and this problem started 2 or 3 weeks ago. Do I need a new battery, or could it be a virus problem, or something else? I don't have an anti-virus program on it and wondered if the bad guys have now got around to PDAs and I should protect it.

It is more likely to be some kind of system corruption. If you have not installed much software and don't stand to lose much if you were to reinitialize the system, I would try a hard reset if all else has failed.
The OS is non writable, so it would be very difficult for anyone to hack into while you are online, unlike a desktop or laptop system, which is far more vulnerable to such attacks. These PDAs are however vulnerable to power surges and power cuts when connected via the cradle etc. I would also agree with others that the battery is certainly a weak point with these models, although personally I am not willing to pay out HP prices for a replacement each year, and mine also tends to show 80% charge much of the time.
You should also probably be more specific about your description of the fault - under what conditions does it crash, for example?

Not a virus.

Go into settings and check how much free memory you have. Without enough memory machine will crash.