i have both xp and ubuntu in my system...i installed xp in d because c had bad sector...when i am going to install ubuntu it takes d as extended partition under which /,/home, swap falls. it takes c as primary by default...i tried custom partition to make / as primary but could not do it.. so how can i do it...

when computer start , start menu shows that xp is in /dev/sda1 i.e c but xp is originally loaded in d....so how can i do it....

You do not need / as primary for Ubuntu to work. Linux can boot from an extended partition as well. So you do not need to worry about it.
Even if it does not show / as primary your Ubuntu should work just fine.

It is hard for me to understand what you are trying to do. I assume that by "c" and "d" you mean two seperate physical hard drives? In Windows, seperate hard drives and even partitions are named with different letters, so C: and D: can both be on the same hard drive. In Linux, there is no such designation ac "C:" and "D:". Did you mean /dev/sdc an /dev/sdd (and therefore you have four drives)?

Were you trying to make a new primary partition? you can only have a small number (I think 4?) primary partitions. All of the others are logical partitions.

When you say the the "start menu" are you speaking of the boot loader? Did you install a boot loader? If you use GRUB, you can enter command line mode by pressing "c". You then check to see what the contents are of each drive and partition. Type "help" for instructions.

Sometimes drives are mounted in a different order. /dev/sda is whichever is loaded first, and /dev/sdb is whichever is loaded next. This is not necessarily the same order each time. It is better to specify partitions by UUID.

If you are a little more clear, we might be able to help you more.

i have 3 drives formated as fat32..c.d.and E...in D i have installed windows xp because c has bad sector...now when i am going to install ubuntu it consider C as primary partition and takes D as extended underwhich it makes /,/home and swap partition for unix...but i want to install / as primary partition...so how can i do it ...kindly help me in this regards..

@OP: I'm going to reiterate this post because you seem to have ignored it.

@OP: two members have asked you something which you have completely ignored. Its rude here. Do not do that.
Also, there is an option to make partition primary by force. You need to click the check box.

Insisting on using a disk with a known bad block is perhaps a false economy. It will fail shortly.
Fred.