Some spyware/scareware/malware infections can be tricky enough to block you from updating or installing anti-malware programs. Luckily some of these software developers post links where you can download the new rules and install them manually.
Use these links to download Malwarebyte’s Anti-malware and the latest rules for it.
Offline rules – http://data.mbamupdates.com/tools/mbam-rules.exe
Install Malwarebyte’s first, and then run the mbamrules.exe file to install the latest update.
Depending on how severe the infection is you might have to download these files onto a USB drive from another computer and copy them on to the affected computer and install them.