changing MOBO without losing O/S

If you think about it logically, it is easy.

In most cases you need to install the motherboard drivers after you've installed windows. That being the case it obviously means that the system will run without them, even if some devices don't work.

This means that you should be able to uninstall the motherboard drivers and get back to that initial state. Once you've done that, simply replace the motherboard, install the drivers for the new motherboard and you are good to go.

Just remember to make sure that the drivers for the new motherboard are on a device that will still work once you've uninstalled the old motherboard drivers.

then whats all the fear about when i ask a retailer to replace my MOBO without losing the OS.

ok found out that i have sufficent slot/space on my MOBO for GTX280, so wont be needing to change it.
now i have an 500W PS, ist enough, i have following installed:
intel core duo 2.66
1 DVD burner
1 CD burner(can discard it)
3 Gb ddr2
1 network card
1 External Dvd burner(dont think it needs supply from PU)
1 Dongle
1 Mouse signal receiver
and a printer, again i dont think it take any power from power unit.

Depends entirely on the PSU.
If it's a quality unit, 500w would be just shy of nVidia's 550w minimum recommendation for that card but probably OK.

well then, just to be on safe side ill add up one 800W.
clocker do you have any thought on Epsilon PSU? retailer which am thinking to buy my stuff from says he can install that model, but i never heard of it, right now what i have is an FORCE PSU and am satisfied.

FSP- makers of the Epsilon- also provided the Sparkle brand PSUs that we used as our base units when I worked at a shop, and they were fine.

The power supply is the one part I have always overspent on because it's the one component that can (potentially) fuck up everything else.
Typical failure doesn't usually kill the whole machine but it could.

Since leaving the biz, I have no direct or anecdotal experience with the Epsilon brand so you're on your own.
Presumably your retailer has your best interests- along with his bottom line- in mind, so it's probably OK.

acctually retailers try to sell you what they have in stock 1st then try to find buyers choice/interest.

I didn't.

there is a different between professional technician and a sellers, technician will always think of possibilities but sellers only think about profit. here unfortunately technicians are always kept in workshops and some people who know nothing about technical parts of hardware except if one part will fit on another run the shop.

Actually, I think the difference would be between a big box store- where your assertion about "what's in stock" is most likely true- and a small, independent shop (like where I worked).
We had no insulation between us as technicians and an unhappy customer, so getting it right was not just an admirable policy, it was self-preservation.
I also had- and exercised- the ability not to cave in to unreasonable customer demands/expectations. Chain retailers feed the public's delusions about product and succeed because of volume. The stupider/less-informed the customer, the better off they are.
We didn't have that luxury.

its funny but here its quite opposite, a big chain shop unlike a small shop has assistance not a direct sellers, they are not responsible for branch profit so they most probably will guide you to their best, but in a small independent shop of 1 sales man and 2 technician staff shop one guy would be responsible of selling whom must achieve something or his job is on the line, god forbid if the owner is acctually running the front office, it would be running into mine field . but this guy which i goto is pretty good guy, it happens few times that he doesn't have what i need but calls like 15 different contacts till he finds it for me, where other shop which i used to go for about 5 years never did this.