rebooting and crashing

Every time I boot my computer, I get a message from avast asking me to reboot… “avast! needs to restart your operating system. Do you want to do it now?” Every time I do a virus scan, my computer crashes to the blue screen. What is wrong? How do I fix it?

It is certainly a strange set of errors, whilst there are some things that can be done to check and possibly resolve the asking for reboot, but add to that the crash on scan there may well be more to this than a single issue.

Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
This could have an impact on how avast works (or rather doesn’t).

What Operating System are you using ?
What is your system spec. e.g. CPU, RAM, etc. ?

It may well be best to do a clean reinstall, but that would be best to wait until you answer the above questions.

operating system is XP SP3, processor is AMD Semprom +3400, 2.1 Ghz, 960 MB ram

No other AV installed in the rememberable past

Can you schedule a boot-time scanning?
Start avast! > Right click the skin > Schedule a boot-time scanning.
Select for scanning archives.
Boot.
If infected files are found, it’s safer to send them to Chest instead of deleting them.
This way you can further analysis them.

Did you try a repair ?

If you did and try another scan, if it blue screens, please try to gather information from the BSOD, like the Error number and in most cases there is a Title, like NOT_EQUAL_OR_LESS_THAN in that style of CAPS with underscores_between_the_words.