The first message is normal, since NT Option Pack was made before SP5, so disregard it.
The second message includes a reference to a URL reference to a Microsoft KnowledgeBase article that says the server probably had a corrupted install of BackupExec. See if it is listed in Add/Remove Programs (if it is, uninstall it), and if not, then see if Veritas has a KnowledgeBase that tells how to remove corrupted installs of their product. If not, you may have to format and start over.
In regards to getting Exchange 5.5 installed (to include OWA), get the most recent version of NT4 installed (including SP6a) BEFORE installing NT Option Pack. Then get ALL the NT4 and IIS4 Security Patches downloaded, then take the server offline (remove the NIC Cable). Then install NT Option Pack, rerun SP6a and install the security patches. When you install NT Option Pack, it makes that server a 'virus/worm magnet' and I've seen several instances when it gets infected before the patches get installed. When IIS4 is up and running, then install Exchange 5.5 (custom install to make sure OWA is included), and then install Exchange 5.5 SP4 and the most recent security patches.
If you want to make the process alot easier, and you have Windows 2000 Server available, then install Win2k Server and SP4 first, then install Exchange 5.5 on there. Ex5.5 runs much better on Windows 2000!!
Hope this helps!
DDrew