If your company does a lot of working with contractors or independent entities, chances are that they have policies in place to support people who aren't using institutional systems. Check and see if there's a contractor policy already in place that covers this and if any other employee has opted for this freedom.
After a couple of years of frustration with super-crappy work machines, I checked with my employer (a university). Was there anything for which I needed their hardware or software to access? The answer was no. I don't do financials, I access institutional data at only one step above the general public (Read-Only, limited access) or through portals that are already designed to work off-site.
So I cut the cord and don't use a work-provided machine for anything. It's occasionally annoying (as when my HD died and I had to deal with that on my own) but in so many other ways, intensely liberating. I watch colleagues wrestle with clunky "hardened" laptops or the large Powerbooks they get if they're not stuck with a low-spec desktop. I attend meetings with all of my documentation and data-crunching done on a netbook or ereader that's customized to my workflow. Plus, because I have consulting and contract work outside my full-time job (with employer's full knowledge and consent), my tech is even partially deductible at tax time.
If you can't use your own or can't afford to at this point, talk with IT about the acceptable policy for occasional private use and software add-ons they'd approve. At least you'll know you'll be in their good graces when you're on the road for them and would like to surf to /.