how do i scan for malware or a virus

Please specify did you see these virus alerts in your browser? if yes - it is definitely scam. As far as I know there is no viruses for mac. If you see this warning in your browser you may try to scan for adware using Thomas Reed`s adware medic tool.

Here is the link: http://www.adwaremedic.com/

If you have problems with the start up you may try to run your mac in the safe mode

- Click on the Apple sign() at the upper left corner of the Desktop, select 'Shut down' from a drop-down menu;

- Wait until your computer turns off, then press the Power button(⌽);

- Immediately after you hear the startup tone, press and hold the Shift(⇧) key. It should be held as soon as possible after the startup tone, but not before the tone;

- Release the Shift(⇧) key when you see a grey Apple sign() and the progress bar below;

This will force the system to perform several maintenance routines and load essential system software.

Once you've seen the Desktop, start a Disk Utility scan to detect and repair file system errors:

- Open Disk Utility application:

1. Click on Finder at the bottom of your Desktop;

2. Select Applications folder from Finder's left sidebar and then open Utilities folder (or press Command(⌘)-Shift(⇧)-U), double-click on Disk Utility icon;

- Choose your main hard drive from the left sidebar ('Macintosh HD', usually);

- Click on 'Verify Disc' at the bottom right corner, your Mac will perform a diagnostic test. Click on 'Repair Disk' if prompted to fix problems, or follow instructions given on the Disk Utility window;

- Once the routine finished, click on ""Verify Disc Permissions"", after it's done, click on 'Repair Disc Permissions';

- Wait until the process's finished and shut down your Mac, turn it back on in 30 seconds;

After this try to hold down the shift key and open Safari. Run adware medic tool

mickfromcoventry wrote:

i keep getting virus alerts when on the internet and yesterday i got a stop symbol and had problems getting my computer to start up

The first is likely to be no more than a tech support scam. There is no virus, it's just a ploy to get you to call a "tech support" phone number or install something nasty. See:

Tech support scam pop-ups

(Fair disclosure: I may receive compensation from links to my sites, TheSafeMac.com and AdwareMedic.com.)

The second is not related to the scam, but without more information, it's impossible to guess what might be causing it. Where did you see a stop symbol? In what way did the computer have trouble starting up? Did it just take a long time, or did it do something else? If the latter, what specifically did it do?