Uninstall One Anti-Virus Program to Avoid Conflicting Commands Between Two

Ignorance is bliss, it is said. But I realise I cannot afford the pleasure of ignorance if my pc is to be protected from constant advancement of virus writers.
My laptop Acer Extensa 5620Z is run on Windows Vista Home Basic.
It has the default Windows Defender and I have installed two:
1... Free Antivirus - BitDefender Free Edition
2... avast! - Download antivirus software for spyware and virus protection.

A friend knowledgeable in IT matters tells me I must have Trend Micro protection immediately. He says it will scan the pc and tell me how many viruses have infected my pc and also how to be rid of them.

Please tell me (1) if I should uninstall either BitDefender or Avast as both are described as ''anti-virus''. Both seem to run in backgraound and download and install new updates frequently, on their own.
And (2) follow advise of my friend and get Trend Micro too.

I have experienced in the past these ''free scanners'' demand money after the scan, to remove the items they have discovered in the pc. And I cannot afford to make any payment.

Please advise me as am confused and in fear. Thank you.