If your credit card is being rejected, try updating IE fully. The store client is somehow tied to IE, and if it's not fully up to date it considers is as a security issue and doesn't allow you to go to the next window.
If your pre-paid RP voucher isn't working, you're inputting the code wrong. try hitting caps lock and numlock if numbers are involved.
If it's your graphics card that's the issue, update all the drivers. If you dont know what that is, go to the website of the company that made your graphics card (nvidia, ATI, whatever) and go to the downloads section. There, provided you know how to use a computer, you'll be able to find the most recent drivers for your G-card.
Hope that helps.
PS: Next time clarify, don't just qq all over the place and expect people to sniff their nails to get a whiff of what you're trying to day. I was THIS close to trolling. I dunno why i chose to help instead of troll. Looks like it's your lucky day! :P