screen size changes

A quick question, I have never had this problem before. When I resize the screensize on my system, it will not stay the same size the next time I pull up a program or website. This is now happening on two systems using XP professional. In the past when I used my mouse to resize a screen, it would stay that size from that point on. Any ideas ? Thanks, Vince

do you just grab a corner and drag? or click the square between the - and the x on the top right hand side?