Windows Vista and Windows 7 protect the Program Files folder from modification. Installing *should* work fine, but updating may or may not depending on the software. You can disabled UAC from your control panel temporarily until after you update, or you can just move everything to a folder other than in Program Files. For example, c:\mirc\ . In Windows Vista, go to Users in the control panel and you can turn it on and off from there. It may require restarting your computer.
If you move your entire current mIRC folder to a new location and then install the update, you should not lose any settings. Settings on newer versions are stored in your AppData folder, so wouldn't be lost anyhow, but it's possible you still have settings in your install folder depending on what version(s) you have used. It's safer just to move the whole folder to guarantee that you won't lose your settings.