I need help with a consumer problem?

I put my laptop computer in a shop to fix an issue with a virus 5 months ago,i was initially quoted �20.00p this went up to �30.00p and then �50.00p, i was at home when the shop called me and said they would have to replace the hard drive as well and this would all be the sum total of �100.00p, and theyi said would you like us to go ahead with this. In had no choice as i never had the full knowledge or know how to fix the computer myself. Once the laptop was repaired i was given a call and came and picked it up after paying the �100.00p, i was told and given a receipt with a 1 year warranty, 4 months later the laptop caught a virus and it shut access to windows down therefore disabling the computer,i went over to the shop and told them about the virus and wether my warranty would cover this,straight away i was told it did not and it would cost me more � to fix the virus, the reason the virus infiltrated the laptop was due to the internet security failing, this was the same security that was installed and guarunteed by this shop. I was really angry and said a few cross words to the man behind the counter, and walked out of the shop and was walking away when the owner came out on the street and shouted ,do you think you are a hard man?, i said no but you are a con man. I proceeded to the local library and gave the shop a bad online reveiw. 1 hour later i was called by the police who asked if were told i only paid they could meet with me and talk regarding the laptop matter, we met and i explained my situation and how i had paid �100.00p and was angry at the customer service from this shop, the police said that the shop was only paid �50.00p, i insissted that was not true and that it was �100.00p i had paid and had a warranty etc, the olice said they were not taking sides, but one of the officers said he could arrest me for this,they said i should go back into the shop and apologise,and i refused,i have since discovered that the hardrive i received was a 2gb, and the 4gb hardrive originally in the laptop was taken out,they told me it was replaced with an identical hard drive,what can i do about this situation?