(what you thinking saying 1.5? you thinking linux floppy again? ooops!

me forget tell sniperchang about 1,8gb recommendation specially for windows disk protection requirement.

will edit.)
Windows Disk Protection Requirements
Windows Disk Protection requires a minimum of 1 GB of unallocated disk space. This unallocated disk space will become the
protection partition—for storing disk changes temporarily when Windows Disk Protection is turned on. Some computer uses—such as burning CDs and DVDs—require large amounts of disk space (double the size of the project being written to disk). Keep this in mind and ensure that sufficient unallocated disk space exists when you configure computers that will be used for this purpose.
Note
An alternative to increasing the size of the protection partition for burning CDs and DVDs is to configure your disk-burning software to place its temporary files off the Windows partition.
To turn on Windows Disk Protection, you must fulfill the following requirements:
•Ensure that at least 1 GB or approximately 10 percent of the
Windows partition (whichever is greater) is available as unallocated disk space.
•The unallocated disk space must
follow a primary partition; it cannot be at the beginning of the disk.
•The disk that contains unallocated disk space may have no more than three primary partitions.
•The Windows partition must be a basic disk. Dynamic disks are not supported by Windows Disk Protection.
Note
The protection partition can also be created in free space in an extended partition, or you can use unallocated disk space on a second physical disk. For more information about each of these techniques, see the “Manage the Protection Partition” at the end of this chapter.
You can use the Disk Management utility to view the current partitions on the hard disk.
http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/sct/scthch02.mspx