virus definitions updates?

[aQrose]

My avast is updating the virus definitions each & every time I start my computer which is a real PITA when I'm trying to get online to get something done quickly. It really makes my system crawl.  Like for instance, when I started my computer this morning, it ran & updated them and took for-freaking-ever and prevented me from getting something in by a deadline. I finally got to do what I needed to do & turned the computer off. A few hours later, I got back on & it did it all over again. 
Is there a way to tell it to just run the updater once a day or schedule a time that would be more convenient for me for it to run? Or tell me there are updates & ask if it's OK to download & install them?
Thanks in advance!

[DavidR]

How frequently do you connect to the internet ?

Unless the frequency of your connecting exceeds the VPS update check duration (4 hours) then it shouldn't be connecting every time you connect.

Some information about your system would help:
Operating System; CPU; RAM; your connection type, dial-up/broadband, etc. ?

[aQrose]

I'm online off and on throughout the day, numerous times often for hours each time. My system is old, XP, Pentium 4 2.5 Ghz, 1.5 GB RAM. I have AT&T U-verse internet

[DavidR]

If you switch your system off between usage, that may be what is triggering it to check again.

Whilst your system may be old there are many with much lessor systems without issue, so generally it should be up to the task. You could check what other programs are allowed to run on boot and be severe on what you allow (security based software, system services, absolutely essential programs only).

How long have you had avast and or was this previously OK ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?

The Update check interval is 240 minutes, this can be increased - avastUI > Settings > Updates - 'Auto-update interval' section and change the value, increase to say 480. That however may, not help on the first check after boot, if not there is another option which we can try if required.