used pc needs help

Reinstall the OS is the safest bet
You can spend a lot of time uninstalling, creating new profiles (if you have the admin PW) running AV and multi anti-spyware apps. But, the best bet is to reinstall.

The above answer is absolutely correct but only if you have the Install Cd's that should have come with the computer. Since the previous owner has not seen fit to remove their data from the computer you are free to use it as you like so if they have Internet Banking you should have free access to their Bank Accounts or Credit Cards. But to be on the safe side I would be wiping the HDD with a DoD approved Wiping Utility and then reloading but only if you have the Cd's which you haven't mentioned.

If you didn't get the Cd's you can create a new user account with admin privileges and then change accounts and delete the original account along with all the files in that account while it will not get rid of everything it will remove most of it.

As for the pop ups you need to run various Spy @ware programs the most commonly used are Ad aware available from

http://www.download.com/3000-2144-10045910.html?part=69274&subj=dlpage&tag=button

Spy Bot S &D available from

http://www.safer-networking.org/en/download/index.html

and the MS Anti Spy Ware Tools available from MS at

http://www.microsoft.com/downloads/details.aspx?FamilyID=321cd7a2-6a57-4c57-a8bd-dbf62eda9671&displaylang=en

You will also need to run a couple of AV Scans over the system as well so you can download AVG for free at

http://www.grisoft.com/doc/Programs/lng/us/tpl/tpl01

After you download install and update all of these programs you can then run scans over the system and clean out any infections that these programs find. Although they are likely to get most of the infections there is no guarantee that they will get them all but if you do not have the means to reload the system this is your best alternative.

Col

unless the xp installation cd came with the used computer, you will have to buy a copy.
once you have the installation cd, you simply startup the pc with the windows xp setup cd inserted in the drive and follow the screen instructions to delete existing partitions, create a new one and install xp.
run windows update immediately and repeat until no more critical updates are available.
if you received any cd's that say 'recover cd' or 'os' on them like you might with a branded pc, such as an hp, compaq dell or gatewy, try starting up the computer with the cd installed.

Depending on the model of computer....

I buy used HPs and Compaqs, and they usually have a hidden partition with the OS on it. If it does, you will likely see a tag on the computer itself with the product key on it. If you have the product key on the box its legal. Most of these comps didn't ship with install disks, but HP will send them to you upon request, free except for shipping.

If thats the case, you need to know how to access that partition (F10 on startup for HP) and then use it to reinstall. The HP program has the option to wipe out everything and start fresh or repair. You want to wipe everything clean.

If this is an option, I would add some cautions.

The partition will likely not have the latest version of the OS. With another PC, get a copy of the latest OS patches, copies of anti-virus software, anti-spyware, updated drivers etc., and write it to a CD. Install this stuff right after the OS, and before you connect to the net. Otherwise you are in for a world of pain....

James