What is it and how do I get rid of it?
First update your Windows via windows update.
Then restart in safe mode and scan with avast and delete all files found infected. Trojans and worms can not be cleaned only deleted.
As MacLover said get to windows update NOW.
These are some of the exploits that win32.gaobot can get in through.
W32.Gaobot.AFW is a worm that spreads through open network shares and several Windows vulnerabilities including: The DCOM RPC Vulnerability (described in Microsoft Security Bulletin MS03-026) using TCP port 135.The WebDav Vulnerability (described in Microsoft Security Bulletin MS03-007) using TCP port 80.
The Workstation Service Buffer Overrun Vulnerability (described in Microsoft Security Bulletin MS03-049) using TCP port 445. Windows XP users are protected against this vulnerability if Microsoft Security Bulletin MS03-043 has been applied. Windows 2000 users must apply MS03-049.
The UPnP NOTIFY Buffer Overflow Vulnerability (described in Microsoft Security Bulletin MS01-059).
The vulnerabilities in the Microsoft SQL Server 2000 or MSDE 2000 audit (described in Microsoft Security Bulletin MS02-061) using UDP port 1434.
Exploits the Microsoft Windows Local Security Authority Service Remote Buffer Overflow (described in Microsoft Security Bulletin MS04-011).
The worm also spreads through backdoors that the Beagle and Mydoom worms and the Optix family of backdoors install.
W32.Gaobot.AFW can act as a backdoor server program and attack other systems. It attempts to kill the processes of many antivirus and security programs.
Treat the problem not the symptoms, get updated otherwise you will be doing this again and again.