I have a machine that is on my network & internet. Some few minutes ago I realised that the automatic antivirus scan had gone off. When I launched the virus scan, I noticed that the automatic scan had gone off/was disabled. I tried enabling it but in vain. What's the cause of this? Might tis still be another virus or trojan?

Almost certainly. You might want to try a full manual scan and the use of another virus checker like the free Grisoft AVG http://www.Grisoft.com to track it down. Also use Spybot - Search & Destroy from http://Security.Kolla.de and Ad-Aware 6.181 (or later) from http://www.LavaSoft.nu. Update all programs with the newest reference files before doing a scan.