Very hard to tell what may be going on, since the possible "changes" may have occurred at either of two points when the computer was out of your sight and control.
1. When the original user reclaimed possession temporarily and installed "some" software.
2. During the time your Mother has had the machine.
There is some contradictory information in the course of your description.
--Windows XP Home - does not suddenly become XP Pro, simply with the installation of SP2.
--The source of the XP Pro + SP2 CD needs to be questioned even if both came from you.
----If the XP Pro Operating System (OS) has already been installed and activated on a previous machine, problems can happen
--A single licensed copy of XP Pro can be used on one machine only.
--The Microsoft CD of SP2 is appropriate to use on either Home or Pro OS and will not change the OS from Home to Pro
--Even the downloadable SP2 can be burnt to CD and used to update Home or Pro.
--Having your Mother uninstall SP2 should have neither helped nor hurt the situation, though you'll want to get that machine back to running with SP2 and all the critical updates and patches as soon as possible for security reasons.
You didn't mention AntiVirus and AntiSpyware protection, so it is "possible" that some bad-guy has infected the machine since it has been in your Mother's possession. (freezing due to new software or hardware installation could be malware)
Seems like you will be having a chance to visit your Mother, since it will be difficult to assist you in further helping her without direct and firm information about what is happening with the machine. (for instance, specific/exact Error Codes)
Yes, if you XP Pro CD is legimate, not installed on another machine, and not an OEM version, you should be able for get the machine running with a Repair Installation or a format and Full installation.
If the XP Pro CD is from a no longer used, store-bought branded machine, it will very likely "not" work for this different machine now in your Mother's possession.
Sorry you plan to do something nice for your Mother has turned into a headache for her and a disappointment for you.
Let us know if you have any additional specific information about the machine's condition.
There "might" be specific procedures that can be pursued.
But "Most Likely", you'll be better off with a fresh OS installation.
Here's a link to the well-regarded Fred Langa's Non-Destructive Total Repair Rebuild of Windows:http://www.informati...cleID=189400897
The above link may turn out to be "just interesting reading" in your situation, as a format-reinstall may be much more direct for you.
When you perform a Format and fresh Installation, remember to take all of the following steps, in order:
Presuming you have Windows Installation media (legit CD's with product ID Key that can be activated)
--Chipset Drivers (specific to the machine)
--JAVA Runtime Environment
--Install SP2 if not included in the initial Windows installation
Defrag the Hard Drive at this point
--Install any additional PCI/peripheral Drivers
--Install any peripheral hardware (printer/scanner/etc)
--Install desired Application Software