Their are two main methods to make change in registry either you manually go and make changes in the directory or write the path with string and dword value on note pad and save it with .reg extension when registry will create you can add it to original place by just clicking on it.

some commands are given below... chek it. It works for me hope it works for you also.
To open folder option when it is hide by virus

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer]
"NoFolderOptions"=dword:00000000

To enable regedit if it is disable by virus

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Group Policy Objects\LocalUser\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"DisableRegistryTools"=dword:00000000

To see hiden files if unhide button is clicked

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Folder\Hidden\SHOWALL]
"RegPath"="Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\Advanced"
"Text"="@shell32.dll,-30500"
"Type"="radio"
"ValueName"="Hidden"
"DefaultValue"=dword:00000002
"HKeyRoot"=dword:80000001
"HelpID"="shell.hlp#51105"
"checkedvalue"=dword:00000001

To enable task manager if it is disable by virus

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System]
"DisableTaskMGR" = Dword:000000000

even many more techniques are their you can change in the dword value to take its -ve advantage.
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www.onecomputerguy.com

Hear you can get all the tricks not only about Windows XP even for Windows Vista Windows 7 and many more......................