These are very similar. "They have installed software" (the 2nd example) means the people you are talking about (the subject, "they") installed the software themselves. In the first example, "They have software installed" means that the computer has the software on it, but is not specific about how the software got there. "They" might have installed it themselves, or someone else may have installed it.

Please note, neither is a sentence. if they have security software installed . = their device/computer has security software installed on it - however the phrase can apply either to the people or the computers if they installed security software. - The person you are referring to did the installation themselves