Ouch! So they planted malware on her system and god knows what else!
First before we worry about her system make sure you contact any credit card and/or banks if she does any buying or bank transactions online. As they likely have her info!
I would also make sure she alter all of her passwords for anything she uses on her system once the system is back online. As well as her email account and any other online service. If you can I would recommend you do this sooner than later!
OK, so you have a few issues here as this is a very old system so you need to use the older OS's that still work on it. Hopefully she still has the CD/DVD that came with the system (grey disk) or has one of the retail versions of the OS which had pictures of different Tigers & Lions. You'll need that to boot up the system.
The next issue is cleaning out the malware and/or spyware thats on her system. This will be tough as again this is such an old system you need a program that will run on it. The other approach is not try to save the system but instead reformat the drive which will wipe out everything!
In either case I would strongly recommend you backup anything she cares about (pictures, stories she has written) onto a USB thumb drive so you can check them from your system using a Windows based antivirus/malware/trojan/spyware app.
The other approach is finding someone with a Mac with a FireWire connection that you could connect to your mothers system in Target Disk Mode (you'll need a cable to interconnect the systems). Here the version of what your mothers system is running doesn't matter as the app would be running on your friends system again using a good Mac based antivirus/malware/trojan/spyware app. To do a good search & destroy.
You'll also need to train your mother (sorry I had to do the same thing). You tell her not to give out any info to anyone calling her. Have her get the persons name & phone number and give it to you so you can do a reverse lookup to check it out.
Even a phone that shows caller ID can be faked! So it can't be trusted either anymore I ended up taking my mom's caller ID phone way from her and gave her a simple POT's phone instead which I programed with the basic numbers she would need.
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