You're going through what I learned a long time ago through trial and error. Aloha version of 5.0x and XP do not always play nice. Some 5.0x keys I've gotten work with XP, and some I haven't. I can't find any rhyme or reason to it. I think it may have something to do with the hardware in key itself (I've probably tried it 10 different times with 10 different keys).
As it stands right now, we more or less have a standing policy that customers who have 5.0x software will get Windows 2000 when it comes time to replace their server (and since most of them haven't upgraded the Aloha version, they're generally don't care what OS is on there, as long it works). Like you, I prefered to use XP, mainly because it's easier to install in terms of drivers. The reality is that there is not much difference functionally once the server is built, whether it's XP or 2k. In fact, I sort of like the fact that 2k is less user friendly, because there is less of chance of user trying to install iPod software, use MSN Messenger, or connect their handheld device.
I know most Aloha dealers preach the same thing I do-it's a file server for your POS system, so let it do just that. If you want to run other 3rd party apps, better to get a Dell for $500 than take a chance on mucking up a machine that's central to your business. So with that logic in mind, it really doesn't matter what OS is on your PC--2k will be just fine.
If you're really a glutton for punishment, you can download the Alladin HASP drivers the other people metioned in the post. Just google "Alladin Hasp driver download" and you'll get results for such links. Since you already have the OS installed, it's worth a shot (it did work for me once!)
Hope this helps. Best of luck!