When you installed PM, it should have given you the oppertunity to make a set of "rescue" floppies, which can perform most PM functions. If you have them, or can create a set on another computer, run PM and see whether it sees the partitions you expect, and whether the parition with XP is marked as "active". The back of the PM manual (a PDF file these days, not usually a paper manual) discusses errors and some ways to fix them, including using the floppy set. I recall that the PM floppy set can mark a partition as "active". If not, a program called FDISK can do that, as can Norton's GDISK (which comes with some Norton products).
http://fdisk.radified.com/
If using FDISK, be sure to get the latest versio for large hard drives:
http://support.microsoft.com/?kbid=263044
Usually the first partition on the first internal hard drive, also called "C:\" is where XP is located.
However, 99.99% of XP can actually be on any partition. But, a few key files must be on the first one.
ntdetect.com
ntldr
boot.ini
The first two can be copied from the XP CDROM, or from any other PC running the same verison of XP. BOOT.INI is a simple text file that can be make in any editor, or copied form another PC and modified as required. The XP recovery console can make you a new BOOT.INI, if you determine that is the only problem.
Links about the XP recovery console:
http://www.wown.com/j_helmig/wxprcons.htm
http://www.xxcopy.com/xxcopy33.htm (near bottom)
http://www.kellys-korner-xp.com/win_xp_rec.htm
http://support.microsoft.com/kb/314058
BOOT.INI must correctly point to where the bulk of XP is installed. A typical BOOT.INI looks like this:
[boot loader]
timeout =15
default = multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS = "Microsoft Windows XP Home Edition" /fastdetect
NOTE: “disk(0)†is the primary hard drive and “partition(1)†is commonly known as “C:â€.
NOTE: If SP-2, add “/NoExecute=OptIn†so the last line reads:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Home Edition" /fastdetect /NoExecute=OptIn