ntkrnlpa.exe This is a core component of windows it may be corrupted or infected. I suspect it is not the cause of the crash but a under lying driver that is crashing. From searching on the web for other problems with the same exe file there are many types of crashes.
This may not fix it use slimdrivers and update your outdated drivers one ata time and and see if this has any effect. Always create a restore point for each driver updated slimdrivers will ask you everytime before it downloads.
If you have a virus infection in windows XP before the upgrade than it an still be there. You could use the windows 7 disk and have it repair the system see if that will fix the ntkrnlpa.exe BSOD.
Here is a guide to using the installation disc as a repair disc
http://www.techsupportalert.com/content/how-repair-windows-7-system-installation-disc.htm