Where does a boot virus affect?

Are there more than one boot sector on a harddisk or it doesnt matter the partition?

There can be several boot sectors on a HD.
Typically, if several not-very-evolved (such as MS-DOS or Windows 95/98) OS are installed on the same HD, there is one primary section for each OS, and one boot sector for each OS + a boot manager such as LILO or GRUB.
The MBR contains the bootstrap loader which loads the boot sector of the active partition (the partition of the boot manager for instance) and executes it.
The boot manager will allow to launch specific OS (for instance LILO and GRUB know how to load & boot a Linux kernel) or generic OS, from the boot sector of a primary partition or a logical partition.

If you have only one OS, then you probably have only a single boot sector.
If you have several OS, then you probably have several boot sectors.

Thanks for this advice, and is it thanks to code guru too!

It is a little too late now. But for the next time you could try this command:
fdisk /mbr
It rewrites the master boot record.

Such people should exist in Kenyan roads.

iagina