Losing passwords & login details

[#1]

Hi
Often I find that websites I regularly use (including these fora and PN's own site) request me to re-enter login details even though I have already set the site to remember my details and log me in automatically. This can be frustrating and a nuisence.
I understand that some spyware, etc. programs cna delete cookies but not sure which of those I run do this.
I use SpyBot S & S but the websites in question are included in the 'Ignore cookies' option. I also run Adaware 2008, Malwarebytes & AVG Free 8. I run each every day but it seems that I don't have to re-enter passwords, etc after everytime I run them.
Does anyone have any suggestions as to resolve this - can a failing boot drive also cause this? The reason I have asked is that this has happened before just before a boot drive failed but maybe it was coincidence?
Thanks

[#2]

what browser are you using - Firefox surprisingly is not secure in this respect and login detais are in Tools Options  leave it up to you to discover where, or would you like me to PM

[#3]

I use IE7 (just downgraded from IE Beta 2 - tried it and it needs too much attention before it's usable IMHO)
But these problems appeared before I installed IE8 anyway.
Cheers