I want to replace vista with xp, but When I tried to install os it is booting from cd but it is not recognising harddisk

There are special partitions on the hard drive that are recovery partitions and they say what software can be and what will boot on the system. Since your laptop came with Vista, this partition has to be deleted for you to be able to use this disc on another computer or if you want to change the operating system to something else then what it came with from the factory. Below are the steps to take to enable this.

EISA Partition Delete

1. Open a command prompt as administrator.
2. Run Diskpart application by typing Diskpart in the command prompt.
3. In the “Diskpart” prompt, enter rescan command and press Enter key to re-scan all partitions, volumes and drives available.
4. Then type in list disk and press Enter key to show all hard disk drive available.
5. Select the disk that contains the partition you want to remove. Normally, with just 1 hard disk, it will be disk 0. So the command will be:
select disk 0
Finish by Enter key.
6. Type list partition and press Enter key to show all available and created partition in the disk selected.
7. Select the partition that wanted to be deleted by using the following command, followed by Enter key:
select partition x
where x is the number of the EISA based recovery partition to be removed and unlocked its space. Be careful with the number of this partition, as wrong number may get data wipes off.
8. Finally, type in delete partition override and press Enter key.
Once the partition has been deleted, exit from Diskpart, and now users can use the much familiar and much easier Disk Management tool in Windows (diskmgmt.msc) to manipulate the freed unallocated partition. Users can create a new volume (partition) with this space, or simply merge it to existing partition by extending the size of the existing partition.

Its because of sata hard disk
you have to download sata driver from the website
and copy in to one floppy
connect floppy in to the computer
during boot from the cd
press f6
select which version
follow installation steps

Hi follow instruction go to>cmos setting with F2 Or Delete then advance >IDE Configuration> set compatible then save settings and try