But.... my computer is now running so slow its ubelievable ... what have I introduced to cause this.
Having two resident scanners installed is not recommended as rather than provide twice the protection it can cause conflicts that could leave you more vulnerable.
This could cause conflict as they both fight for control over what AV locks a file prior to scanning it, this could cause the slowing you mention.
Unless you are talking about AVG Anti-Spyware, which isn't an anti-virus but an anti-spyware program. avast should have detected AVG and may disable elements to try and avoid conflict.
What should you do, uninstall the second AV, AVG anti-virus.