Since 2 weeks to 1 month ago, my PC suddenly turned slower. Specially at boot up sequence: after the booting selection, Windows logo appears with a slot where a short blue bar is moving from left to right, this usually takes about 10 sec for me if not less, now it takes a whole 3 min or more, like if my PC was waiting for something to start up fully.

After booting up, in desktop, every application seems to require more time to get "stable", the red LED of H.D.D. just light on each 2 sec where the whole PC seems to "think" a bit (mouse slowed down, with lags while opening applications, games, etc). And I have 3.76 gb free on the drive partition where Windows XP is installed.

Maybe you can try this,
Use at your own risk. These are XP Tweaks.
Disable XP Boot Logo
Be aware that removing the splash screen will also cause you not to see any boot-up messages that might come up (chkdsk, convert ... ), but if your system runs without any problems then it should not matter.
1. Edit boot.ini
2. Add " /noguiboot" right after "/fastdetect".
(or check the /noguiboot switch in msconfig on the boot.ini tab)
Upon restarting, the splash screen will be gone. It can be re-enabled by removing the new switch.
Boot XP Faster
First of all, this tweak only apply to those who only have one HDD on their primary IDE channel (nothing else on device 0 or 1) and a CD-ROM and/or DVD-ROM on the secondary IDE channel. Each time you boot Windows XP, there's an updated file called NTOSBOOT-*.pf who appears in your prefetch directory (%SystemRoot%\Prefetch) and there's no need to erease any other files as the new prefetch option in XP really improves loading time of installed programs. We only want WindowsXP to boot faster and not decrease its performance. Thanks to Rod Cahoon (for the prefetch automation process...with a minor change of mine) and Zeb for the IDE Channel tweak as those two tricks, coupled together with a little modification, result in an EXTREMELY fast bootup:

1. Open notepad.exe, type "del c:\windows\prefetch\ntosboot-*.* /q" (without the quotes) & save as "ntosboot.bat" in c:\
2. From the Start menu, select "Run..." & type "gpedit.msc".
3. Double click "Windows Settings" under "Computer Configuration" and double click again on "Shutdown" in the right window.
4. In the new window, click "add", "Browse", locate your "ntosboot.bat" file & click "Open".
5. Click "OK", "Apply" & "OK" once again to exit.
6. From the Start menu, select "Run..." & type "devmgmt.msc".
7. Double click on "IDE ATA/ATAPI controllers"
8. Right click on "Primary IDE Channel" and select "Properties".
9. Select the "Advanced Settings" tab then on the device 0 or 1 that doesn't have 'device type' greyed out select 'none' instead of 'autodetect' & click "OK".
10. Right click on "Secondary IDE channel", select "Properties" and repeat step 9.
11. Reboot your computer.
WindowsXP should now boot REALLY faster.
visit http://tweakxp.com
You might find your answer there.

I have two HDD's and DVD Drive and a CDr drive will this tweek be ok?