I guess you need to remove the remnants of your previous antivirus http://singularlabs.com/uninstallers/security-software/
Then repair avast from "Add/Remove Programs".
You can also delay the first update after reboot, delay avast service after reboot, and/or change the frequency of updates' attempts.
In addition, if you have all your bandwidth being used (like for example in P2P applications), then the update will take "some" bandwidth, some time and some other resources.
In short, this is not the usual behaviour in all and every system, so something in your system is conflicting with avast, making your system slower than usual.