Hi I have recently purchased a Laptop with Win 8 on it, and my ISP Dodo sent me a new usb modum to run with this.  I still have a desktop running XP, and to use the internet on that - I have to swap my sim card from new mobile usb to the old one.  I would really like to have both machines run of one modum, is there a way of doing this?First thing you'll have to do is speak to your ISP - Most providers that I have seen do not allow doing this.There are a few ways of doing this.  First of all is to setup internet connection sharing so that the laptop connects to the internet via the USB modem then SHARES the connection with the desktop.  Another way, if the desktop has wifi abilities, is to install SOFTWARE like http://www.connectify.me/hotspot/ which would allow the desktop to connect to the laptop via wifi.  Another way would be to find a compatible broadband router that would accept the USB modem you have and create a hotspot (and allow wired connections as well) that both the laptop and desktop could connect to.  Here's one example: http://cradlepoint.com/products/small-business-home-office-routers/mbr95-wireless-4g-3g-router.  You would need to determine if it is compatible with your particular modem.

I used a similar setup a few years AGO in order to provide internet to my house.  Had a USB modem plugged into a Cradlepoint router and had multiple computers connected via ethernet as well as additional computers connected via wifi.  It worked but multiple computers sharing a single 3G connection meant perfromance wasn't the best.  You should be OK with just 2, especially if you only use one of them at a time.Just a note.
A cell phone provider has a 3G or 4G network. It is not the same as Wi-Fi.
May mobile devices can be used as a "hotspot"  when using the 3G data network. For help with this, you contact your cell service provider, not the ISP. Unless the ISP is also a 3G provider.
It is not the same today as it used to be.
Reference:
http://www.bbwexchange.com/wireless_internet_access_providers/
3G, Wi-Fi, WiMAX, and Wireless High-Speed Internet ACCESS