Well I would upgrade if possible, and I you know you said it was not an option, but I would sesiously contemplate upgrading for many reasons. So if that is not an option then I would make sure that WMI is functioning. You can try my repair script
Though this is not necessarily the "LANDesk, Inc. Sanctioned" method, you may be able to get by with simply upgrading the inventory scanner files that go down to the client from the SP5 disk. The inventory scan client is frequently updated between service packs, meaning that there probably is (was) a hotfix out prior to SP5 if you want to fuss with LANDesk support about it. If not, try to download the SP5 client via security and patch and replace the LDISCN32.exe. I don't remember if SP5 for 8.7 was where LANDesk changed the way the inventory scanner wrote hotfix information into the database schema, in which case my suggestion won't work, but it is worth a try. The inventory server service would have changed in SP5 as well, which could also be a minor issue, but again, its worth a shot if you can test this in a controlled environment. The notable files used by LANDesk inventory are:
Download the patch and copy those files to an offending machine --if the scanner doesn't blow up AND the inventory updates in the database, this should fix your issue. If this does fix your problem, you can overwrite the ones in your ldlogon directory on the core with these and it'll prevent future agents from having the issue. As far as the existing clients, you can build a package or a custjob script to push this.