Everytime I try to complete the Thoromir quest and fight Argamir and his mate in the tomb the imperial guard turn up and kill me for resisting arrest. I have no stolen property. I don't remember this happening when I played this game before. Just replaying pre Skyrim release...and getting very annoyed!! Help please

Do you have the latest patch installed? Have you used any cheats? Something like removing your infamy, can corrupt the bounty system.

I wonder if it is a patch needed. I have just reinstalled my original game but not any patches. Will have a go at that. Many thanks.

Did you sneak-attack Argamir or Rolgarel? The game considers that to be an assault and will send guards to arrest you. The quest is broken at this point and you'll have to reload to a previous save.

Yes, you have to enter the mausoleum and let Agarmir give his little speech and attack you. If you sneak attack him, or if you turn around and leave the mausoleum, and then reenter, you'll trigger bugs.