I finally did it! I'm not exactly what step made it work so I'll just list out what I did. I did the regular things first like uninstalling the old driver, shutting off, adding the graphics card, and then trying to install the driver but it kept giving me the same error. My computer also wouldn't recognize my graphic card as the AMD Radeon. On the devices manager (Computer>system properties>device manager>display adapter) it only saw it as a standard video graphics adapter and it kept installing a default driver so I right clicked it and uninstalled it but after I restarted it, it turned back into Video Graphics Adapter. Then I looked up somewhere that you could stop it by right clicking on computer > properties > advanced system settings (in the upper left) > Hardware Tab > Windows Update Driver Settings > change it to "ask you everytime". ( at this point may I also say that I had the driver software downloaded from this site on my desktop http://support.amd.com/en-us/download/desktop?os=Windows+Vista+-+64. I then went to device manager and tried to update the Video Graphic adapter and I just had it browsed to the (Computer>HP>AMD) folder. It still wouldn't work. I then booted up the computer on safe mode (press 8 when the computer is just beginning to turn on but before the windows emblem pops up) I went to update it again on the same folder - failure again. I then went to programs and features and fund I still had an NVidea driver for the ethernet. I uninstalled that and (without restarting the comp) updated the display adapter and it automatically recognized the graphic card as the AMD Radeon HD 5450. I restarted on normal mod and it worked. The Nvidea driver was probably blocking the install I guess. Anyways in short if you get to the same point as me
1.) Download this software onto desktop
http://support.amd.com/en-us/download/desktop?os=Windows+Vista+-+64 . (If you do not have the AMD folder in Computer>HP then open it) It doesn't matter if the installation fails as long as you have the AMD folder under HP
2.) Go to Computer>System properties>Device manager>Display adapter>VGA graphics adapter and right click uninstall and click the box to delete it. If your computer starts to reinstall the adapter go to device manager find it while its installing and right click and update it. Click browse folder and browse it to the AMD folder under Computer>HP> (if it fails its okay)
3.) restart the computer on safe mode (F8) when the computer turns on and choose safe mode (just keep tapping F8 when the computer starts to turn on if you're not sure how to enter safe mode)
4.) If it starts to reinstall the Display adapter do the same thing in step 2
5.) Go to programs and features, search for drivers and unistall the Nvidea driver only.
6.) update the driver like in step 2 and it should work.
7.) Shut down the computer and turn it on normally.
(although the thing preventing the install was probably the Nvidea driver in step 5, I' listed out what I did just in case I'm wrong)
After around 4 days of googling for help on tom's hardware, going to best buy, and other sites it finally worked! Good luck to anyone who encounters this problem and hope this helps.
(By the way Best Buy told me that my Psu was not enough but they were wrong and I would have had to spend a lot more money for another psu and probably still have the same problem)
Also here are my system specs again. (If your system matches mine or is close to mine and you have the same problem then this may thread may help you and I hope it does)
hp slimline s3720y
Windows vista 64-bit
Processor: Pentium(R) Dualo Core CPU E5200 @ 2.50GHz
4 GB RAM
320 watt PSU
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