Urgent Help Needed! Hidden Porn Downloader

I have a laptop, issued for work, that I also use at home. Recently, while this computer was on the network at my job, it spent two days downloading huge amounts of pornographic videos. The site that it downloaded the videos from is blocked by my employer's filters, but somehow, the computer downloaded porn. I am about to lose my job over this but am being given an opportunity to try and change the minds of the powers that be. I have not been allowed to look at the hard drive, but it has been gone over by the IT department who can find nothing. Does anyone have any ideas of any programs, legal or otherwise, that could have done this, but deleted themselves or are maybe masquerading as some other program? I may be given a few minutes to look at the computer and want to make the best use of my time. Any help that you can give me will be greatly appreciated. I am at my wits end here and need as much information as I can get.

yes there are many programs that can download porn. what do you mean IT cant find anything? software leaves tracks all over the place. maybe your laptop is compromised, surely a half way competent IT person would suspect this first, there probably looking in the add/remove programs panel?huh! rootkits can go undetected from traditional AV\malware apps. deny it, tell them there computer must be compromised somehow.