The problem sounds like you are trying to delete something while using it, which is not possible.
Next time you start the computer, when it shows āpress ESCā or āF1ā, do so. It should say it during the start-up screen. If you miss it, then you will need to wait and restart again.
Once in set-up, look for something along the lines of āpartitionsā or āformatā, and take a gander at your options. Think logically on what makes sence to do. If done correctly, you should be in a safe DOS mode running off the motherboard (CMOS) and this will allow you to destroy and rebuild partitions and restore hard-drives.
You dont neccessarily WANT to destroy partitions though, this can lead to numerous problem. I did this with my old computer and fried the system. Good thing it was 11 years old, i need a new computer anyways.
A different solution to the destroying partitions route would be to overlay one OS ontop of the previous one. Like re-recording an MP3. Iām not sure exactly how this would be done.
The floppy boot-up is always going to be your back-up. That will always work aslong as your system is not fried (it could happen).
However it just dawned on me that you wrote that you are NOW running a dual-boot system, where both OSās are XP. Are you given a choice upon start-up of which to boot up. Because if you are given that choice, then it should be quick and easy to just enter the new OS that you just installed, and delete the partition of the drive holding the old windows system.
In any scenario, it is unlikely that the system would tell you that another parition cannot be deleted after installation, because the self-installer automatically deletes the installation files after its completion.
However this scenario COULD potentially arise if say you hadnt fully installed the OS, or something was missing in the installation.
I hate fukcing computers.
Good luck with your endeavors,
(Try and pad of paper and pencil from now on)