I am not sure how you would completely lose 'Computer' in the navigation pane so badly that it's even gone in 8. It's possible you could have acquired a bit of malware that removed the link at some point. Did you ever find any malware on your system with your anti-virus in the past?
I've seen similar things, when cleaning up and repairing other peoples' Windows machines that were infected. For instance, I ran across one machine whose desktop wallpaper was completely removed, start menu programs all completely erased (including the windows default ones), and locked out access to most of the harddrives until the malware was removed. Even when removing the malware, the start menu hierarchy was permanently lost.
The point is, some malware is designed to 'hijack' your system, making parts of it inaccessible until you 'pay money' to have it 'repaired'. It's very possible something similar took away your computer navigation item.