There are 2 English patches, one for the UK and North American versions, and one for the Australian version (still at v1.45).
Are you shutting down all non-essential programs (especially anti-virus) before trying to install the patch?
The patch checks the registry to find the install path, and will give that error if it doesn't find what it expects. In Windows XP, did you install the game from an account with administrative privileges? Have you used any registry compression or cleaning tools, that may have been a little too aggressive?
If that is the problem, you could backup any saved games (just in case) and re-install the game. Another method to add/change the required entries is to use a registry file, described in
this post. For a much more user friendly interface, you can email Larian support (support@larian.com) to get a vbs file (Visual Basic Script) that can make the appropriate registry changes.
