No so thing as XP "2003". I am assuming you have either XP Home SP2 or XP Pro SP2.
Can you see what the DOS screen says? What if you boot into Safe Mode?
Please clarify "continuous flash". That is a confusing term for me, sort of like a "constant moment". Do you mean the DOS windows opens and immediately closes over and over again and never stops? Or does the window opens then immediately closes once
every boot?
Note that some programs use a similar looking "splash" screen when loading.
Are there any errors shown in Device Manager?
McAfee 8 is an anti-virus program, have you scanned for other types of malware? You might want to run through
Cleaning Out Malware for a complete cleaning.