These are .reg files that can be applied to modify the user agent.
IE 5.0 User Agent
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent] "Version"="MSIE 5.0" "Platform"="Windows NT 5.0" [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform] "SV1"="" [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\Version Vector] "IE"="5.0000" [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent] "Version"="MSIE 5.0" "Platform"="Windows NT 5.0" [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform] "SV1"="" [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Version Vector] "IE"="5.0000"
IE 6.0 User Agent
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent] "Version"="MSIE 6.0" "Platform"="Windows NT 5.0" [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform] "SV1"="" [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\Version Vector] "IE"="6.0000" [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent] "Version"="MSIE 6.0" "Platform"="Windows NT 5.0" [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform] "SV1"="" [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Version Vector] "IE"="6.0000"