files changed to bitcrypt, money demanded, reinstall help needed'

My computer had a virus that was caught by micro security essentials. After deleting the virus via security essentials I found my text and picture files inaccessible and having a bitcrypt ending with a message asking for money to unencrypt them. Security essentials didn't find any sign of the virus scanning after the initial deletion however on using google I came to the solution of reinstalling the op system as many suggested the virus leaves a backdoor?

I used the reinstall disk from dell and followed the instruction from system restore. When finished I had no wireless connection and whilst looking at the drivers files found bitcrypt files in the new drivers folders dated the day before reinstallation.

I have two external hdd connected one shows no sign of infection the other has text and picture files infected but not all( im not sure whether this is from the virus or if I transferred them when moving video files before the reinstall). I also now have an windows.Old file of 98gb size.

I've since written a number of text files which remain unaffected but am not convinced the virus has gone and would ask for any advice. I am most worried about the external drive as it has over 2gb of video files which I would like not to lose. As to the reinstallation apart from losing the wireless lan its the 98gb file that worries.

my comp
dell xps 8000
windows7 64bit home premium

Thanks for the reply. The virus scans show no sign so I am assuming its safe. One question with the windows.old file can I delete it as it takes up a fifth of the hdd.