How to check startup items

I have recently received a package by e-mail from a somewhat trusted source and opened it without giving it much thought.
To my great surprise, nothing happened (I think I saw a window opening for a second and nothing more), even after another couple of attempts at opening it.

I am now worried the package itself might have been a virus/trojan/keylogger: are there any ways to check whether something weird has been added to the list of programs/services run at startup (like Windows's msconfig)?
If this can help, I didn't get an administrator password request prompt.

Unfortunately, I have deleted the suspicious file, so I won't be able to scan it; also, I'd much rather not have to install an antivirus just to check the system this once: I have my data already backed up on an external hdd and am prepared to format if necessary.