Dimension 4100 - Invalid Boot Diskette error received randomly

I am getting an error message Invalid Boot Diskette Insert BOOT diskette in A:  when I start the computer which is a Dimension 4100 running on Windows 98 working great for the past 5 years till couple of months.
I first got this message about 6 to 7 weeks back. I tried to log in with an valid boot diskette but still no use. I tried playing with the set up  options but still no use. Everythime I got the same error message. Then suddenly about two weeks after I saw that message, I tried to start the computer and computer started without this error message running the Scandisk with a message that my computer was not shut down properly. It was working fine for couple of days unitil there was a total power failure at my place. When the power came back and when I started the computer, I got the same error message.
I thought may be there was some hardware problem. So opened the system, took out the hard drive, cables and put it back again. Still I got the same error message. Day before yesterday after about 10 days, when I started it, I did not see the error message. I was working the whole day and shut it down that night. Yesterday when I tried to start, I see the same error message.

I have tried disabling the floppy disk drive from the set up. Still same error message.

I did reformat my hard disk several months before using the dell resource and recovery CDs.

Any one seen this error message? Can any one please guide or explain what I need to do? It's puzzling to me as why it shows the error message sometimes and sometimes it is not.

 

I appreciate your help.