Hello,
Sounds a bit like a missing driver for the XP operating system. Win 8.1 has "native" support for Wi Fi with Realtek driver packs as part of the Win 8.1 operating system. With Win 8.1 Wi Fi is simply "plug and play".
XP will support Wi Fi but does not usually have specific drivers or "one size fits all" drivers for the sheer number of Wi Fi "dongles" available.
Do a search for drivers for:-
[name and model of your pocket Wi Fi thingy] and XP (32 bit I assume) and see what pops up. There are many "generic" drivers Wi Fi and XP out there and a process of trial and error will find the right one for you.
Also, with any Win Operating System the Wi Fi driver software will want the code or serial number of the main modem/router. If this is not printed on the device or in the manual or somewhere run a search for "WEP" and see what it finds. The Wi Fi "dongle you have looks for this "connect code" so it can talk to the rest of the hardware. It may be an idea to include the name/model of your laptop in the search as well.
So, for XP it seems that the Wi Fi card installed in your laptop needs the correct driver software.
The Wi Fi card and supporting driver/software in the laptop looks for the code or "address" like a phone number in basic terms from your pocket Wi Fi to connect to.
All going well, you will have "net" on your laptop.
Good luck searching and experimenting and I hope you succeed.
One good thing about XP. XP is great at supporting/recognising newer hardware and driver software.
Cheers,
Mark. (Aunty Jack).