Improve older single core laptop by o/s upgrade from winXP to win7

No. Win 7 uses a little more resource than Win XP so performance will actually decrease a bit. This is especially so if you do not disable Aero in Win 7. Aero is basically some graphical enhancements to the Win 7 experience and it can be somewhat intensive for an old and weak graphic card / core.

Your best option is probably a clean install of Win XP and search around to find out what background programs can be disabled. I kinda understand how you feel, I have an almost 10 year old IBM ThinkPad T40 (Pentium M 1.5GHz) that has gotten slow over the years because programs have become so much more bloated. Even surfing the web is somewhat slow because of all the Flash banners and Java code running in the background. But install Win 7 is out of the question since the T40 only has a DX7 graphics card.