you'd have to edit your boot config.
I don't know if your using lilo.conf or grub.conf.
I also don't know that much about linspire. There maybe a gui configuration to make it easy.
But I expect that your using Grub, just a guess. And your going to have to edit a file normally, I don't know about linspire though. But it should work.
I assume that your able to select from a grub menu at boot uptime to select linspire OR select windows.
edit /boot/grub/menu.lst
and look for a line that says something like this:
default 0
That tells which menu entry is default.
The numbers start with 0 being the first entry, 1 being the second, 2 being the third menu entry, etc etc. It counts like a prorammer does, starts from zero instead of one.
If you don't have a default line then 0 is the default boot entry.
To set the default, change the number to the corrisponding entry you want to be default, and save the file.
If your using Lilo then it would be a bit different.