slow down of my machine

my machine sometimes slows down when login and sometimes freezes when working. i have checked for malware and found nothing

A Pentium 4 with only 512mb RAM? The minimum specs for W7 32-bit are 1GB so it is not surprising. Your machine is barely functional with W7.

I agree with TM, Windows Vista didn't even like a machine with less than 1GB memory. My personal recommendation for Windows 7 is at least a dual core CPU with at least 1GB memory. Thats not all, I wouldn't recommend Windows 7 if video capabilities didn't support Windows Aero (Although I confess this is a cosmetic issue I have).

Downgrade to XP Porfessional or Linux(The only OSs that work fine with 512 mb ram.)
Or change the CPU,upgarde the ram to at least 2 gb.That means you have to change the whole motherboard.(you havn't said which MB you have.)

I wonder how your pc runs in windows 7 with 512 mb ram.(the minimum for 32 bit is 1 gb and recomended is 2 gb and the maximum is 4 gb).

Yes Windows XP would be better to run on that machine. Linux is not for everyone and would have a huge learning curve.

This is not the first time I heard of W7 running on less than 1GB but the problems mentioned of the system dragging at times are why no less than 1GB are recommended. There is just to much page thrashing with the virtual memory for it to run efficiently.