i bought a corsair 430w psu and it doesnt fit my case .... maybe because its old .... the space for psu is small... the size of my old psu is almost the size of the corsair's fan... what do i do ? help !
You don't really have too many choises right now, either exchange the PSU for a smaller sized one or get a new case.
Since you have an older case, I'm guessing you aren't into modding and light shows (like me ). There are many low priced cases on the market that will fit the PSU for under $50 if you're not picky. I personnaly own an Asus Vento case which costed me like $30, it's just fine! I have a Coolermaster Silent Pro 500W modular PSU and 2 x 120mm fans installed in it and it's silent. I would go with a new case.
If you have a "pre-built" computer (HP, Dell, etc.) you'll have to do some research first. If you are only changing the PSU you may be forced to buy your replacement from the systems mfg (and that will be expensive). Some of the builders have proprietary PSU's which would require you to use their specific units only. If you don't have a pre-built it will still be easier for you buy a new case than to find a PSU that will fit yours. At least your selection of cases will be greater than the PSU's that will be available to fit your case. If you tell us what you have we might be able to help.