Good question. I went and checked my folder. I have my server wide set to backup daily and keep 3 back-ups, and this appears to be where they are stored.
I browsed around a bit and I didn't see any really large files that seem leftover from Feb 2009 like you have...
I remember talking with a LP tech who wasn't all that impressed with the Plesk backup manager at the time. I remember I had some sort of issue with the Plesk backup manager failing and breaking my Plesk console, and he had to go in a manually remove some backup files that was confusing it.
That was awhile ago...and an older version of Plesk.. but I wouldn't be surprised if it was about Feb 2009
