Slow startup problem

My Dimension 4600 (XP Home) has developed a curious startup problem. When the computer is first started or restarted, the blue startup screen stays on for about 2 minutes before the Welcome screen appears (this step used to be much faster). When the desktop finally appears, everything seems to function normally, except the keyboard. Each key has to be pressed for at least 3 seconds before it responds. Only the Num Lock key responds instantly to the touch. After approximately 20 minutes after restart, the large Microsoft icon briefly flashes on the screen, and the keyboard begins to act normally, with no indication of any problem. When the computer is restarted, the same behavior is repeated.

The Administrative Tools/Event Viewer shows the following errors:

- Event ID: 7001. The SSDP Discovery Service depends on the HTTP service which failed to start…

- Event ID: 7001. The HTTP SSL service depends on the HTTP service which failed to start…

- Event ID: 7000. The HTTP service failed to start…

The Administrative Tools/Services shows the following information:

- The SSDP Discovery Service status: Stopped. After clicking on Start, the following warning appears: Could not start the SSDP Discovery Service on local computer. Error 1068: The dependency service or group failed to start.

- HTTP SSL service status: Stopped. After clicking on Start, the following warning appears: Could not start the HTTP SSL service on local computer. Error 1068: The dependency service or group failed to start.

- Security Center service status: Stopped. After clicking on Start, the following warning appears: Could not start the Security Center service on local computer. Error 1083: The executable program that this service is configured to run does not implement this service.

- The RPC Call and RPC Call Locator service status shows that they are both started. However, running the cmd routine produces the following result:

C:\Documents and Settings\Emil>sc query RPC

[SC] EnumQueryServicesStatus: OpenService FAILED 1060:

The specified service does not exist as an installed service.

I tried the various fixes suggested on the Microsoft Web site, as well as repair reinstall of Windows from a disc, but to no avail. The available dates on the System Restore calendar do not correct the problem. In msconfig/Startup, I removed all but the needed programs (all of which were there before the problem started).

The Dell Warranty Support Service suggested a complete re-install of Windows XP, but I am reluctant to do this (the computer works just fine as long as it stays on).

Thanks in advance for any help.