Hi Siva, welcome to NST.
Sorry if my previous response was too brief, but I was on my way out and I didn't have time to write any more, so I thought a brief note was better than no reply at all.
The wiki
here contains all the information about booting XP with Vista.
Essentially, XP uses NTLDR to boot, which refers to the boot.ini file for information on the location of the XP windows folder.
NTLDR cannot boot Vista, so in a dual boot Vista's bootldr sits at the top of the boot chain, and if you ask for XP, bootldr must chain to NTLDR. Since NTLDR uses boot.ini to locate XP, Vista is in a catch 22 situation of not knowing where XP is, but needing to hand control to it.
It gets over its dilemma by insisting that the files needed to boot XP must be in the same partition root as its own bootldr.
If Vista installed its own bootldr on the XP partition, (like on my system), that condition is already met so the boot is fine.
If Vista installed its own bootldr in the Vista partition (most of the time), then the 3 files will need to be copied from XP.
The reason why my previous short reply stressed the "active" partition, is because that's how you know which of the above 2 conditions applies.
Look in disk management at which of the 2 partitions contains the active flag, and that's the one that the Vista bootmgr was installed to and where everything else must be too.
If you've lost your XP boot files altogether, NTLDR and ntdetect.com, which are not system specific, can be copied from
here, and you can follow the links from the XP page for advice on constructing a boot.ini file from scratch to point to your XP partition.