Freeze apparently NOT from malware!

For several weeks I have had the following problem on an XP SP3 system:

When the machine is hibernating, sometimes (but not always) my waking it produces ONLY my full-screen backgound (a tiled .bmp); no icons, no start bar, no systray.

Moving the mouse moves the cursor, but there is no other response from the system: right-click does nothing. No keyboard press (including ctrl-alt-del) does anything. The only way out is a hard reboot.

Recent scans (AVG, PCTools Spyware Doctor, Spybot S&D) have shown nothing worse than tracking cookies, all of which were cleaned. And PCTools reports that my hijackthis log file showed no malware.

FYI: At initial boot the same screen appears and remains for about 1:50 before the desktop loads.

Other problems which may be relevant:

On boot up I get a Windows alert telling me that ActiveX has been "disabled on this page" - and Spyware Doctor scans report ActiveX has been quarantined and not scanned.

Also, apparently BrowserDefender Toolbar (part of Spyware Doctor, I think) has been disabled (perhaps by one of my tweaks) and I now get an alert every time I open an instance of IE (no matter how may others are already opened) telling me of failure to open update service.

Anyone have an idea about what might cause this lockup?