OK, that is what happens when I transfer sysdm.cpl from one of my XP PCs to the other. If I click Open it opens System Properties. However, when I open it via (for example) Control Panel, this Open/Cancel box does not appear.
It seems that XP somehow takes exception (in some circumstances) to .cpl items coming from a different PC. I am going to investigate further, but in the meantime copy your original sysdm.cpl item back into C:\Windows\system32\dllcache and C:\Windows\system32
(in that order). You did take a copy as I advised didn't you?
Once you have done this just try getting into System Properties again and reply with the results. I will return.
I know that you have said that the PC is showing no malware but has it had an infection in the past? You could try running
MBAM and SAS freebies as a double and triple check. Both are highly thought of by fellow contributors on this forum. You can always remove them afterwards if you want.
Tricky
EDIT: Try rebooting before restoring your original and reboot again after restoring the original