computer freezing

[mxyzptlk]

My PC at home has started freezing when first powered up, usually within a minute of the desktop icons all being loaded but certainly within first five minutes of use. Mouse and keyboard totally ineffective.
Only option is to power down and power up again and then it seems to operate perfectly normal.
I've done a system restore back a couple of weeks but this has made no difference.
I'm running XP and avast sweep does not reveal anything untoward.
Any ideas peeps?
thanks

[TTG]

Will it boot up in safe mode ok? If it does, use msconfig to disable anything you do not need (search on the net how to use it as I don't have time to explain).

Download, update and run freeware program malwarebytes (full scan).

[mxyzptlk]

Done all that over weekend and still no difference, but tks for advice