It's not exactly just a boot menu, during your installation attempt the volume/partition boot record was replaced by a WinXP one that references Ntldr instead of Win9x based IO.SYS.
As the fastest cure you can edit the boot.ini file crated by the WinXP install and change 'timeout=1' e.g to 'timeout=5' to have a 5 second timer. You can also change the 'default=multi..' part to 'default=C:\BOOTSECT.DOS' to boot your previous Win9x OS by default.
But the final solution would be to restore your Win9x boot record by running 'SYS.COM C:' from a Win9x boot floppy/CD where SYS.COM can be found.

Boot to DOS command prompt and type:

1fdisk /mbr

fdisk /mbr was the first thing I did try in win98 after the installation failed, but that did nothing to the boot menu.

I'll try to check out if using sys command will work.

Fdisk /mbr replaces the master boot record. AFAIK in case of an already partitioned disk XP install does not replace mbr (but also XP and Win9x mbr's are compatible anyway). So Instead of restoring mbr you have to restore the volume boot record that actually boots the OS on the active partition. And this is what SYS.COM does among others (like also copying system files).
(The BOOTSECT.DOS file made by XP install also contains the copy of earlier volume boot record and not master boot record)