Connect to internet through usb on dial up computer?

So my grandfather passed away some years ago. By chance I was going through some things and saw his old desktop tower. I have set it up and it works well, runs Windows XP Home Edition. The computer however only connects to the internet via dial up connection. I was trying to tether my iPhone with a USB to get on the internet that way but can't set up a network that works. Any help with getting connected without the use of a Ethernet cable is welcome thanks.

I don't get the bit where you say it only connect to the internet via a dial up connection

It connects by any way you want so long as your ISP supports it

you want to connect to dial-up then get a usb modem

you want to connect to broadband then get a usb broadband modem
(but you are far better getting a router (cable or ASDL) and usb/ethernet or usb/wireless adaptor)

You want to connect to an existing network then get a usb/ethernet adaptorr

you want to connect to an existing network (with Wi-Fi) via wireless then get a usb/wireless adaptor

You'd have to install iTunes as only it has the drivers for the connection. Alternatively look into creating a small ubuntu partition (20gb) as it has support for the iPhone hotspot right out of the box. You could also buy an ethernet-usb cable and connect the ethernet wire to that. Or get a Wi-Fi antenna (20-30$) and connect to your iPhone over Wi-Fi.

If you have a router and modem set up for your other computers then all you can use a Wi-Fi connection with a usb or a card if the computer has a slot to upgrade.

XP was in use before broadband became generally available and used dial-up by default and so your PC probably does not have an Ethernet socket, just a 'phone one.
To be able to use it with a broadband router you need to go into the Internet settings and enable the "never dial a connection" setting, or it will always try to do that;
http://ask-leo.com/why_does_my_computer_keep_wanting_to_dial_a_connection.html
Then you will need to get a USB Wi-Fi adapter to connect the PC tower to a Wi-Fi router if it does not have an Ethernet socket.
That's about all there's to it really, regards, Bob.