Open regedit and nevigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
In right-side pane, u'll see 2 keys:
RegisteredOrganization
RegisteredOwner
U can change their values to change the name, which u gave during Setup!
PS: This is for Windows XP, In windows 98 simply nevigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion