This might help anyone still using XP:
My PS2 mouse quit working after a software install corrupted the drivers.
Although I have System Restore enabled, it did not work because of automatic updates added since creation of the last restore point (another good reason for not using auto update).
To recover, I used the keyboard to navigate to Control Panel --> System --> Device Manager and deleted the mouse entry.
Then, using another computer, I created the following text file, giving it a "reg" extension and copied it into the registry (again, navigating with the keyboard).
=============================
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96F-E325-11CE-BFC1-08002BE10318}]
"Class"="Mouse"
"UpperFilters"=hex(7):6d,00,6f,00,75,00,63,00,6c,00,61,00,73,00,73,00,00,00,00,\
00
@="Mice and other pointing devices"
"Icon"="-2"
"Installer32"="SysSetup.Dll,MouseClassInstaller"
"NoInstallClass"="1"
"TroubleShooter-0"="hcp://help/tshoot/hdw_mouse.htm"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96F-E325-11CE-BFC1-08002BE10318}\0000]
"EnumPropPages32"="syssetup.dll,PS2MousePropPageProvider"
"LocationInformationOverride"="plugged into PS/2 mouse port"
"InfPath"="msmouse.inf"
"InfSection"="PS2_Inst"
"ProviderName"="Microsoft"
"DriverDateData"=hex:00,80,62,c5,c0,01,c1,01
"DriverDate"="7-1-2001"
"DriverVersion"="5.1.2600.0"
"MatchingDeviceId"="*pnp0f13"
"DriverDesc"="PS/2 Compatible Mouse"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96F-E325-11CE-BFC1-08002BE10318}\0001]
"EnumPropPages32"="syssetup.dll,PS2MousePropPageProvider"
"LocationInformationOverride"="plugged into PS/2 mouse port"
"InfPath"="msmouse.inf"
"InfSection"="PS2_Inst"
"ProviderName"="Microsoft"
"DriverDateData"=hex:00,80,62,c5,c0,01,c1,01
"DriverDate"="7-1-2001"
"DriverVersion"="5.1.2600.0"
"MatchingDeviceId"="*pnp0f13"
"DriverDesc"="PS/2 Compatible Mouse"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96F-E325-11CE-BFC1-08002BE10318}\0002]
"InfPath"="msmouse.inf"
"InfSection"="HID_Mouse_Inst"
"InfSectionExt"=".NT"
"ProviderName"="Microsoft"
"DriverDateData"=hex:00,80,62,c5,c0,01,c1,01
"DriverDate"="7-1-2001"
"DriverVersion"="5.1.2600.0"
"MatchingDeviceId"="hid_device_system_mouse"
"DriverDesc"="HID-compliant mouse"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96F-E325-11CE-BFC1-08002BE10318}\0003]
"InfPath"="msmouse.inf"
"InfSection"="HID_Mouse_Inst"
"InfSectionExt"=".NT"
"ProviderName"="Microsoft"
"DriverDateData"=hex:00,80,62,c5,c0,01,c1,01
"DriverDate"="7-1-2001"
"DriverVersion"="5.1.2600.0"
"MatchingDeviceId"="hid_device_system_mouse"
"DriverDesc"="HID-compliant mouse"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E96F-E325-11CE-BFC1-08002BE10318}\0004]
"InfPath"="msmouse.inf"
"InfSection"="HID_Mouse_Inst"
"InfSectionExt"=".NT"
"ProviderName"="Microsoft"
"DriverDateData"=hex:00,80,62,c5,c0,01,c1,01
"DriverDate"="7-1-2001"
"DriverVersion"="5.1.2600.0"
"MatchingDeviceId"="hid_device_system_mouse"
"DriverDesc"="HID-compliant mouse"
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CriticalDeviceDatabase\*pnp0f13]
"Service"="i8042prt"
"ClassGUID"="{4D36E96F-E325-11CE-BFC1-08002BE10318}"
===============
After rebooting, my mouse was back to normal.
The usual caveats about messing with the registry apply.