Yes, this isn't something one should actually do for any reason. I'm writing this up because not everyone installs other browsers and sometimes they think they need to check to see if a site is doing User Agent checking.
To configure user agent spoofing for Microsoft Internet Explorer (MS IE), run regedit expand and select: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent
file>export give that a name e.g. backup of hklm ie useragent
If there's a folder ("key" in registry parlance) under User Agent named Pre Platform or Post Platform, rename them to e.g. -Pre Platform and -Post Platform respectively.
expand and select: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent file>export give that a name e.g. backup of hklm useragent
If you're running Windows XP SP2 then you will have a Post Platform folder (key), rename it and any Pre Platform items to -Post Platform and -Pre Platform respectively.
whatever you do. if you had a Post Platform, you must create a Post Platform key, so right click User Agent and select new>key this should give you: My Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\New Key #1 name it Post Platform
expand and select: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\5.0 If you have a User Agent key, i'll be very surprised. The reason to use this instead of the other one is a technical politeness, you're generally not supposed to make global changes :). If you do have a User Agent key: file>export give that a name e.g. backup of hkcu ie useragent and rename it to -User Agent now create a new key by right clicking "5.0" and selecting new>key
expand and select: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent
I'm not sure what the UA Tokens key is for, i didn't search. since it didn't show up in my ua string, i'm ignoring it.
Instead of actually writing steps for setting the keys, I've decided to provide registry fragments. You should of course read them before using.When you're done, don't forget to delete the keys you created (either through importing or by really creating them) and to rename the dash variants back to their original names. You don't actually have to do the renames, you could instead merge our backups, but you do need to delete the keys that were merged in first, otherwise you end up with some really unhappy things. Thankfully it's easily fixed by deleting the keys and reimporting your backups.