A computer I'm working on has a DUN profile set to dial in to some
router for the purposes of reaching an internal machine, say,
x.x.x.103. This router assigns me some IP on the same subnet, say
x.x.x.107.

I'm on a LAN, so I don't want all traffic to go through the dial-up
interface; I only want traffic for x.x.x.103 to use the dial-up
interface. So I say in the DUN profile configuration not to "Use
Default Gateway", and I made a little program to determine the IP
address that the router assigns me and to add a routing table entry
for x.x.x.103 that uses x.x.x.107.

The problem is that lately (maybe after some automatic update from
Microsoft), DUN is adding some crazy extra routes after it dials up. I
didn't ask for any routes, and I don't want any routes, but DUN is
making four routes. For example, it creates one for x.*, where x is
the first byte of the remote subnet. These routes can't be deleted. If
I say "route delete 10.0.0.0", route tells me that the route can't be
found, despite the fact that "route print" just showed me that it
does.

When I dial up, I don't want Windows to make any routes. I want to
handle the routing situation myself. How do I get DUN to stop adding
crazy routes?

Windows XP SP1.

This is likely because the remote-router is sending routes in the PPP
negotiation messages.  Windows is just obeying.

The reason for the 10.0.0.0 route is rooted in the old classful
routing scheme.  In a classful routing model, 10.x.x.x is a Class-A
address, hence the automatic addition of a CIDR/8 route.

See http://public.pacbell.net/dedicated/cidr.html for information on CIDR.