Hi Farflung
Yes it is possible - there are a whole variety of contracts out there from 1 day a week to cover flexible working to full time ongoing...
I think the best option (if you have no previous office experience) is to try to do a few secreterial type courses before you go to an agency- these can be done online and show you know one end of a computer from the other! (Excel, Outlook, Powerpoint and Word are all favourites).
Also if you want to go into admin I would definately aim for PA/office manager level....I started out on reception many years ago and wanted to shoot myself wihtin 2 weeks...I think if you have a crew mentality you need somthing that keeps you on your toes and challenges the old grey matter.
My job now is pretty self sufficient and I decide how I work and what to prioritise - It can be a bit of a minor nightmare sometimes (think the devil wears Prada) but otherwise its the best decision I ever made!