Code a JavaScript/JQuery home-page alert that shows once per user session

I want to implement a JavaScript alert on my website homepage to alert users that the site uses cookies and that by closing the alert and continuing to browse they acknowledge that cookies are being used by the site.
I have been able to set-up an alert - using the attached script - that achieved this, but it also stopped the page loading; I want the site to be visible underneath the dialogue box. It is not necessary for the background to be greyed out, but it would be better if it is not possible to click on any other items on the page while the alert is open.
I have also looked at JQuery modal dialog solution, which is much neater, but have not been able to implement the once-per-session element. This may be because my site does not run the JQuery cookie plug-in which is beyond my technical abilities, but I would look at modal dialog solution that operated in another manner.
It's an html page, so .asp, .php etc solutions are not possible.
I will pay for 2.5 hours for this job.