The file avg6db_n.dat is a definitions file for AVG Antivirus. It probably cannot be defragmented because it is a "restricted" file...if it could be easilly accessed, a virus could corrupt your scanner. Thus, being unable to defragment is kinda makes sense.
When searching for the file, make sure you enable scanning for "hidden files," otherwise it will not come up in the search results. Go tools->folder options->view->show hidden files an folders. Also enable it using the search preferences prior to searching.
However, I've never seen a definitions file that large...seems REALLY strange to me...mine's measures in MB, not GB! I'd recommend uninstalling AVG again and finding/deleting the file, then reinstall and update...I may be wrong, but it sounds like the file may be corrupt.
Hope this helps,
John
My computers been slow recently so I tried to defragmentate my hard drive, so I used the defragmentator that comes with Windows XP. When I do it it says there is a 4gb file called AVG6DB_N.DAT which can't be defragmentated. I assume this is something to do with AVG Virus Scan, but I have uninstalled AVG and done it again and its still there. I then searched my computer for that file name and nothing came up, and I then went into the computers registry and deleted all AVG6 files from there. Still though it says it cannot defragmentate this file. Can anyone help me?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic