This shouldn't be too difficult to fix, providing you can get some internet access somewhere else...
There are three things you will always need to keep your PC running sweetly - an antivirus package (I know, too late!) - Grisoft AVG is good, and FREE - go
here . Search and ye shall find. Also, download Lavasoft AdAware SE Personal Edition from
www.lavasoft.com and Spybot Search & Destroy from
http://www.safer-networking.org/en/download/
Download all the updates for each of the three programs. Boot your computer into safe mode (F8 at startup) and you should be able to log in. Install at least Spybot and AdAware - you should be able to update these from downloadable files rather than the inbuilt update features, enabling you to do it from another PC... ask if you don't know what I mean. When all is installed and updated, physicaly disconnect your PC from the internet (removephone line or broadband connection) and run all the checks. Remove anything it finds as dodgy.
When all is done, try booting normally.
Let us know how you get on! Good Luck!