Slow computer, low CPU

Hello everyone and thanks in advance. I don´t post for a long time, so let´s see if i do it rightly. I have a Pentium 4 3.40 GHz, 512 MB RAM, a 100 GB disk with only 30 GB used, runing Windows XP SP2. For some time now my computer is awfully slow (not on start uo), mainly while opening a new program or when switching between programs. Firefox takes about a minute to sart, even the explorer is slow and opening a folder tends to be a prety long task to. Nevertheless, the CPU is always runing low in task manager, more or less 8 - 20 %. I have McAfee Internet Security Suite 7 runing (without site adviser and anti-phishing, because of known issues with CPU consumption) and it is up to date and says i am clean, so does SpyBot, AdAware, ewido and blacklight rootkit eliminator. I have defraged the disk, elkiminated start-up junk, cleaned the registry with Abexo. Well, i guess i said it all i could. This is the begining of the HiJackThis logfile: Logfile of HijackThis v1.99.1 Scan saved at 18:37:08, on 12-09-2006 Platform: Windows XP SP2 (WinNT 5.01.2600) MSIE: Internet Explorer v6.00 SP2 (6.00.2900.2180). Thank´s for any kind of help.