You could also get a free 30 day trial of Minitab, which i've been using for a few things
Minitab has much better regression analysis than excel (cause it's a statistics package). So if you're doing other multivariate regression or non-parametric regression, with non -parametric tests, minitab trial would be good.
Otherwise, if it's just ordinary least sqaures, then excel/openoffice/gnumeric would be ok.
On excel you'll probably have to install the data analysis tool, but it's easy to do, as long as you have the office cd!