Mirroring pc hardisk

liddler is right, here's the topic in short:

Create a Mirrored Volume on the Remote Computer
1. In the Disk Management window, right-click unallocated space on one of the dynamic disks in which you want to create the mirrored volume, and then click New Volume.
2. In the New Volume Wizard, click Next.
3. Click Mirrored volume, and then click Next.
4. Under All available dynamic disks, click the disk in which you want to host the mirror, and then click Add. Verify that the disks in which you want to create a mirrored volume are listed in the Selected dynamic disks box.
5. In the Size box, specify the amount of unallocated disk space to use for the volume, and then click Next.
6. Assign a drive letter or path, and then click Next.
7. Specify the formatting options that you want to use, and then click Next.
8. Confirm that the options that you selected are correct, and then click Finish. The mirrored volume is created and appears in the appropriate dynamic disks in the Disk Management window.
9. Quit Disk Management on the remote computer.

YOur choices as far as that goes are to use software RAID emulation (which is what is explained by Guanofan), use a hardware RAID solution (scsi or IDE RAID controller), or use a drive cloning program like Norton Ghost or PowerQuest DriveImage to make periodic backups.  

To make a true mirror, the best method is hardware RAID solution, because with Microsoft's emulation of RAID, all your data can become inaccessible and possibly unrecoverable if or when the dynamic volume information written to the disk becomes corrupt.  With a true RAID setup (if you already have the two hard drives and they are both IDE, a decent RAID controller shouldn't cost you more than $30 US if you shop around online), your data will be safe regardless of what Windows decides to do when it crashes.  I have seen it blow away the dynamic volume info several times, and since that is proprietary, there are really no methods to recover other than what few tools Microsoft may provide.  

THe Ghosting / DriveImage method is the best for data security, because you can store the two hard drives in different physical locations, however it has the con that if you need to recover, you must go back and restore at the point the last backup was made.