Win32.AcidShell is a backdoor program but also has Trojan capabilities.  Its Trojan characteristics allow it to enter your system without detection.  This can happen through a variety of means.  You could have received an e-mail from a seemingly legitimate source asking you to download some software or a document.  The document is infected with a virus that once opened will contaminate your computer.  The virus will execute and run a series of malicious functions that will leave you vulnerable to receiving additional malware.  It is imperative that you remove this pest as soon as possible.
Removal is easier than you think.  SpyZooka is an excellent removal system that will clear Win32.AcidShell immediately from your computer.