As i remember from dual booting with ubuntu a good while back, you got an option screen on boot to let you select which OS to boot into . . . . . . before anything else, and as windows was the primary OS with ubuntu installed with wubi inside it, if you did nothing it booted into XP. So are you getting this screen or not? And can you get to the advanced boot options that show you your installed OS's ?
You should see your two options somewhere:
I'd hazard a guess that something's gone pear shaped with the master boot record but let someone more experienced confirm this, i'm stabbing in the dark here, not keen on dual booting with windows . . . . . . sure something like this happened to me on my XP machine and I fixed the mbr with the OS disc. Don't you have a sticker/badge on the machine or some documentation with the product key ?
Lenovo ThinkPad T61 LM14 & 15 (different HDDs) Intel T7500/2.2GHz/Cinammon 1.6/1.8 Intel GM965. Toshiba Satellite M70: Dual boot Zorin OS & Elementary.Desktop:LM13/15 Geforce 9500 GT, Athlon 7750 Dual-Core 2700mhz,3gb RAM.Linux Registered #562462