Most antivirus software ask you to make a recovery disk, a boot floppy with
some of their tools from which you would boot the system, run the tools and
delete the file. If you did not make one about all you could do is wipe the
drive and start over from scratch. If your file system is fat32 as opposed
to NTFS, you could use a Windows 9x boot disk, boot from it, navigate to the
infected file and delete it assuming that is what you mean.
If the message is just asking to reboot the system in order to complete the
process, then just reboot. Obviously, if it's asking you to reboot in DOS,
then only the above is possible. I can't believe that any antivirus
software made for Windows XP would have booting to DOS natively as an option
in its virus deletion routine.