Device Manager not showing Devices...

The standard answer for this is to
1. Click Start, click Run, type services.msc, and then click OK.
2. Double-click Plug and Play.

If you receive a Configuration Manager message, click OK.
3. In the Startup Type list, click Automatic, and then click OK.
4. Close Services.
5. Restart the computer.

However, since you are also experiencing other errors, I would suspect spyware/virus problems.

First, go into msconfig and uncheck everything except for your firewall, spyware and anti-virus apps. Make sure these apps are up to date (you need two spyware scanners; Spybot and MS AntiSpyware are ok). Reboot into safe mode and run each of these three scanners. Delete any files they find. Rescan with each scanner a second time if they find anything (you want each scanner to come up with no infections eventually).

Hopefully you have caught this problem early enough. Once these problem apps have been on a system for awhile, the only timely option for removal is usually to reinstall the OS.