Computer hangs on bios unless restarted

Hello,
I have a problem with my computer (Compaq Evo D510 CMT, 2.4 GHz, 512MB, 40GB, Windows XP Pro 5.1, sp2) which is every time I press the power button on the system unit to boot it up; it takes an awfully long time –about five minutes- on the BIOS screen but still goes on to start Windows normally. However, if I press Ctrl-Alt-Del while still on the BIOS screen, it restarts (as expected) and goes through BIOS very fast (as it should and used to). If I restart Windows, it does not take a lot of time on the BIOS screen, but if I shut down Windows and press the power button to boot up, it does. It’s like the only way to quickly start this computer is to restart it! And also, when I try to wake it up from standby, it either restarts or the monitor remains off and the keyboard does not respond (i.e. when I press CAPS LOCK, the LED on the keyboard does not go on/off) even though the fans and hard disk spin. It also takes about 40 seconds on the Windows XP boot screen, where initially took only about four seconds. I tried reinstalling Windows but these problems still persist. Everything else works fine, although I did get BSOD about three weeks ago while playing DVD using wmp11 (but it quickly restarted before I could read the STOP message) and it once hung while playing a game (Need For Speed). Could it be something to do with VGA card? But I hardly think so considering the amount of time I spend playing games and no problems occur, whatsoever. I used the DirectX Diagnostic Tool to run tests and all were successful, defragmented hard disk, used Ultimate Boot CD to detect problems but NOTHING. What worries me is that all these problems developed suddenly and at the same time. What could be the problem here? I really need help? Sorry for the long text.