I always choose the auto shutdown when all downloads are complete option in tixati. But the PC shuts down before even completing the full download and this has happen many times and its only when i use tixati.

My specs :
OS :  Windows 7 professional 32 bit
RAM:  4GB RAM
processor: Intel i3 2330m

Log:

The process C:\Windows\system32\winlogon.exe (LENOVO) has initiated the power off of computer LENOVO on behalf of user lenovo\admin for the following reason: No title for this reason could be found

Reason Code: 0x500ff

Shutdown Type: power off

So guys is there any way to solve this problem.

Have you tried setting delay for auto shutdown? Maybe a few minutes delay before shutdown would solve this issue.

Once you close Tixati normally, and wait for the RAM to be freed, that's how long to delay like @Pete says.
Tixati isn't speedy for shutdown. Most likely what happened is tixati did finish but the .dat files weren't saved completely. For my 10K transfers (450 MiB of dat files) I have to wait 3 minutes every time I close it. And this is also waiting for a external drive.