There's a "Print Screen" key remember? Basic hacking/cracking 101.
There are three possibilities:
* One: someone with access to your computer/network installed an actual monitoring program. There are lots of them. Do a search for spyware screen capture, child web monitor, employee computer monitor, etc.
Problem 1: these are genuine programs installed by some user.
Problem 2: some are recognised as "spyware" many are not.
Problem 3: many have options such as hide from system and encrypt or an innocuous name to hide them from child, employee, etc.
Problem 4: if you are on a network some can run remotely.
* Two: someone installed a trojan logger/screen capture program. Again there are lots of them. Do a search for trojan logger screen capture or similar.
* Three: it's all a bad dream. Wake up. It never happened.
A good antivirus/antispyware regime should find and eliminate most illegal installs and some "legal" ones; but not most "user approved" actions. Unless you have such AV programs on auto you should know if such a creature has ever been found. If on auto check back through their logs.
Your mention of long time screen capture with an end date a couple of months ago makes me suspicious of a user installed/stopped/removed application.
The concern is that your passwords and any private info (personal id data, CC number, bank acct access, love letters, etc.) are probably compromised. Of course if its just web design templates check out competitors!
There is no "easy" fix. If future computer data confidentiality is paramount I would backup data files, reformat, re-install applications from original disks, and re-install data files individually after verifying each.
I would also change all passwords, etc. that were ever used over that computer. And monitor for id theft, credit abuse, etc.
It's too hot to be at the keyboard anyway ... get some bottles of beverage, some sunscreen, a friend, and go enjoy the summer.