Grub1.5 Error 21

[(रेवंत) Revant Nandgaonkar]

I had this error 21 while installing 5.10 on my friends comp, I
installed 5.04 and it worked fine.

error 21 was poped up after busybox copied all the other files from cd
to harddisk and asked me to remove InstallCD and  reboot.

what is the problem here? how should i correct it? Now even if i know
what the error is, I cant solve the problem on my PC,

because its only with 5.10 installation, 5.04 worked fine without any
grub errors on the same day!

Ubuntu