I have an old phone that runs as a modem on older versions of windows and gives me great internet access using the three.co.uk internet access bolt on.
I have a new laptop that came with Vista Home Premium.
I also use Ubuntu Linux for most of my day to day work.
Normally, these are not compatible!
However, www.vmware.com's free VMPLAYER allows me to run an older version of windows that runs the mobile phone (an LG U890) as a modem and share it's connection using internet connection sharing with the host system.
I have to say that this combination actually appears to work better under Linux - the vmware player seems more responsive.
It does have some quirks that are a pain, but not enough to stop me using the system.
Because of the way the LG software handles its modem connection, I have to re-share it every time I connect to the internet. Using Windows XP or 2000, this is not a problem (but in earlier versions would have needed the virtual machine to be restarted and probably not work).
As my LG U890 continues to accept and make calls whilst running as a modem(!), I simply fire up the Virtual machine, connect to the Internet using the lg software and share the connection (right click on the network icon and allow other machines to access via this connection) and leave it running whenever I'm at the PC.
The virtual PC also needs a software firewall and virus checker - just to be safe.
You do need a licensed copy of Windows 2000 or XP and installation disks to install onto the virtual machine - If you have an old machine you are not using then you may have all that you need (although under the terms of the license, you are not allowed to run the same system on more than one machine) If you have no older kit you can retire, it is (just) still possible to buy copies of windows XP.
Currently (Nov 2007) the cheaper versions of Vista license conditions do not allow them to be used inside virtual machines - although the memory and cpu requirements make it impractical on most machines at the moment!