No browser installed, not going on the web -- then yes, your XP will last much longer.
Your only sources of infection then are locally installed media and the infamous around the elbow attack.
How fancy can an around the elbow attack be? Very fancy -- here is just one example.
There was once a D-Link router that ran on Linux that wasn't locked down as far as where it could go in your machine.
Some clever boy out there was participating in attempts to turn a common D-Link router into a linux PC box (and yes, you can do this) and he though, "Why don't I use what I now know to stick a root kit into the wide open Windows drive C: boot partition from the router's open accesses and permissions to the entire machine". And he did.
Mind you, he couldn't play with the Linux partitions as they were all locked down in the kernel, but he could side door into Windows Drive C: while the PC wasn't even technically "on" or if it was "on" not technically doing anything. The router is always on and always connected to the PC and at that time all of its resources. The actual computing was taking place in the router itself (all routers are little computers BTW).
His exploit got plugged by Linux, and by D-Link and by Windows very very shortly.
Also, if you ever agree to go to a site or agree to download ANYTHING no matter how innocent (say a free software tool or a game) then don't be surprised if other things can sometimes come along for the ride.
Clam AV (the free Linux anti-viral) simply says that you need to have it running on your freestanding Linux machine "to protect all the Windows machines that you may eventually touch or share a file with". Linux can't be harmed by much, this is true, but Windows is a notoriously easy target to get to from the Linux side.
So, using Linux to get at the Windows machine is a well known pathway.
Sandboxing (as used by Chrome boxes and books) is another trick to keep contamination from going from Sandbox A to Sandbox B to Windows drive C:
Chrome OS keeps
all activities separate by running a separate complete small fast little red Chrome guy in each locked down separated sandbox. Here they are, see how each little red Chrome guy is standing in his own separate little private sandbox waiting for something to clobber upon.

Chrome OS works like that, very secure and virus proof as if one little Chrome guy gets tricked and overcome by something nasty out there on the web then the other little Chrome guys are not affected by it at all. That one sandbox gets closed and scrubbed, the rest stay open and healthy. Each tab (or open web page) is in its own separate little world.
The overall OS schedules a powerwash (OS full chksum reload) at the very next boot if there is an incident seen in any sandbox. And since Chrome OS is so small and light, this only takes 20-30 seconds to do.
Chrome OS has a very rigorous design and enough layers of backup protection that not much is every going to make it through the layers. And some of these layers of protection are in and actually part of the Google Server Farm structure that does a lot of the heavy lifting in Chrome OS's world.

You might fool a little red Chrome Guy, but can you fool Google's huge constantly updated Server that your program filters through?
Nope, didn't think so.