Edit your C:\boot.ini file CAREFULLY.
for example- I have three possible choices when I boot- XP, XP recovery console, or Ubuntu. my boot.ini is as follows:
[boot loader]
timeout=15
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
C:\CMDCONS\BOOTSECT.DAT="Microsoft Windows Recovery Console" /cmdcons
c:\wubildr.mbr="Ubuntu"
Now, If I only wanted Windows XP to boot, I would change it to:
[boot loader]
timeout=15
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
Since your probably just pressing enter at this prompt most of the time, I'd remove the entry that isn't listed as the default; after confirming that it boots properly into your OS of choice, you can delete the other installation (whose location will be have been listed in the entry deleted from boot.ini.).