Please post your complete system specs to get clear and precise answer.
For the time being, try to run startup repair at least 2-3 times and see if it found and fixes any problem with your system. In windows 7/ Vista, you can access it directly by continuously tapping F8 key just after starting the system and choosing Advanced Startup options and then startup repair from the system recovery options.
In Windows XP, you'll require Windows Installation media either CD or USB and run startup repair from there.
Post back if you need further help.