Your aunt's internet is probably slow because she has a really bad processor or limited RAM. It likely has nothing to do with her actual ISP at all.
This would be my assessment as well... To confirm, check to see how much the computer is using virtual memory. If the hard drive never seems to stop reading and writing, then that's a strpong superficial indicator of low RAM, as the computer is using the HD to act as RAM. Check the maximum amount of RAM your aunt's computer can handle and see how much is in it now. Go ahead and order the maximum, taking into account the upper limits of 32-bit configuration. If you can max the memory out without needing to convert to 64-bit, or if she's already using 64-bit, then that should solve the problem, or at least improve performance significantly.
I apologize if I am telling you stuff you already know. But you would be surprised by how many simply don't think about this sort of thing and are quick to blame the ISP for something that their own machine is causing. I was in that boat before I started working for a computer parts store and learned how to build computers and a bit about the more technical side of things. I will never claim to be an expert, but if I have run into a problem before, I can use what I learn to help others...