If a computer freezes, it’s done so for a reason. Letting it sit there usually makes things worse, especially if it’s a hardware problem. If something overheated and you left it on “for a few hours” then it’s likely unfixable now.
If explorer is crashing you’ve either got a virus or a serious corruption issue. Backup everything and reinstall Vista. If that works for a week or so then do a clean install of Windows7.