When I shut down my  PC, the monitor flashes all colors. I have an Acer monitor. How do I stop this only happens when I log off, It never did this before.

Also when I start up the computer,it takes up to 20 minutes to get to Icons, never did this before,/what is happening

Try this:-
At the command prompt type  msconfig  .
Check the   Diagnostic Startup    button and restart.
If the problem disappears check the startup services and startup programs.
If the problem persists check the   Selective Startup   button and uncheck the Load System Services and the Load Startup Items boxes and reboot.
If this solves the problem then play with the services and then the startup items till you identify the issue.

PS Once you have fixed this issue look at Ray's Speed Startup program since it can help with this process.
Cheers