|You can find documentation for the Vostro 220 at http://support.dell.com/support/edo... The "Service Manual" has comprehensive instructions for replacing the PSU, complete with diagrams.|
It's always worth spending that little bit more on a PSU - go for a make like Corsair, Antec, CoolerMaster, rather than a no-name cheap unit. PSUs are rated by wattage - the existing PSU should have some identification that shows what wattage it is (I believe that the Vostro 220 has a 300W PSU as standard); go for one the same or, preferable, a little greater. If you want one exactly like the original you can find them in various places. Google "Dell Vostro 220 PSU" and you'll get a lot of hits.
Replacing a PSU is fairly simple if you are reasonable competent at mechanical things; if not it might be better to get someone who knows to do it for you.
All this is assuming that your PC is out of warranty - if not, contact Dell.