that use the .PST extension for their files. A search provided the following users: BMail, a bulk mailing program is one -- but I hope you are not a spammer. PST is used for the Lahey Fortran paste buffer -- but this is rare as Fortran is seldom used for programming PCs. The Microsoft Exchange Address Book uses this extension, but the size makes this unlikely. The Ulead Pattern image format also uses .PST; however, their data would be stored as many individual small .PST files rather than one giant one named outlook.pst, as in the case of Outlook.
In Outlook all the personal folders and their individual files, which generally is mostly email, are embedded into a single large file named outlook.pst. There may be dozens of folders and hundreds of files embedded in this single file. That is why it can be very large, but rarely in the GigaByte range. There is one such file for each user of the PC.
You say that you have never used Outlook, so how did its data file get so much stuff in it? Perhaps it was being used without you being aware. I have heard that some other email clients actually use Outlook for their purpose. (BTW, you never answered my question about what client you are using for email.) I suppose it is possible that your machine was hijacked and the hacker was using your Outlook for nefarious purposes. It is too bad you did not look and see what was in these embedded folders before removing Outlook. That might have provided an important clue as to the source of these files.
So, not knowing the source and contents of this file, and with Outlook now gone, it appears to be of no use. However, I would still follow the common procedure for such unknowns -- first rename it (something like outlook.xst or inlook.pst), then if your operations do not need it for a month or so of regular use, delete it. And I would follow Kees' suggestion to compress it to regain some of the drive space immediately. An alternative would be to move it to a DVD, from which it could be recovered if later needed.(I doubt that it could be compressed enough to fit a CD.)
Because of the possibility that this file came from an intrusion, I would run a complete suite of anti-malware -- checking for and removing viruses, Trojans, spyware, and any other baddies that can be found. This forum posting lists some excellent ones you can choose from if you do not already have your own favorites.
Hope this helps
Frank