Everytime I start my computer Microsoft Security Essentials aks me to scan my computer I just wondered if this happens because I have got it sheduled for daily scan.
I guess that would depend on (1) what you mean by "every" time, and (2) if the time of day (or night) are your scans scheduled for.
If you restart your computer several times a day and it scans "every" time you restart it, that would be a problem - assuming you are letting it complete. If you start it once in the morning and it tries to run each time, you may have it set to run at 2AM but can't because the PC is turned off.
Note that Windows 7 by itself is much harder to infect than previous Windows versions so I have mine set to scan once a week, instead of daily as I did with XP. MSE is running full time in the background anyway and if kept current, the scheduled scans are just supplemental.