"My system BIOS reports something like "Primary Master HDD S.M.A.R.T. status bad"
Backup your data and important files immediately! This is an indication that one of your hard disks is physically failing. Anytime a SMART message pops up, the general consensus is - replace the HD ASAP. SMART is usually an early-warning of impending HD failure. The sooner you replace it the better.
As for serial.sys:
Serial.sys
Serial implements the Serial service. Serial is used as:
• A function driver for legacy COM and Plug and Play COM ports Note that a COM port provides a 16550 UART-compatible interface.
• A lower-level device filter driver for a Plug and Play serial device that requires a 16550 UART-compatible interface An example of this configuration is a serial device on a PCMCIA card.
Serial's operation as a filter driver is identical to its operation as a function driver.
Serial features the following:
• Plug and Play, power management, and Windows Management Instrumentation (WMI)
• Power policy owner for a serial device stack that includes Serial
• Supports an unlimited number of standalone COM ports and multiport boards
• Controls interrupts and communication with device hardware
There should have been an error code with the messasge...and knowing your Operating System and the code would be helpful.
I'd haul my computer to a repair facilty, like COMP USA before you cannot get into your computer at all. But, most important, remember to back up those important files!