As your symptoms are non-specific, I've gone for broke. I recommend you uninstall your Malwarebytes' in case it's become corrupted, and because I'd like you to install it as part of this procedure...and if you already had it installed, it would cause additional problems.
Try this:
Firstly, boot your computer to the Safe Mode menu screen. You do this by repeatedly pressing F8 as soon as you boot up. Once there, use the arrow keys to highlight Safe Mode with Networking. Continue to boot from there, by pressing Enter. You will now see some drivers being loaded. There will be a pause at some point. This usually lasts for no more than 30 seconds.
If that’s successful, open your browser, copy and paste this link into the address bar and press Enter. It's a direct download for RKill. Save it to your desktop, then run it. When it's running, your desktop icons will vanish for a few seconds. When the notepad report is displayed, just close it. You may now delete RKill if you wish:
http://download.bleepingcomputer.com/grinler/rkill.exe
RKill SHOULD HAVE STOPPED THE INFECTION FROM RUNNING, BUT IT WON'T HAVE REMOVED IT.
Now open your browser and download TDSSKiller.exe from Kaspersky Lab. It's tiny, and takes just a minute to run. It hunts down and kills a specific family of rootkits.
http://support.kaspersky.com/faq/?qid=208280684
Regardless of the results, open your browser and copy and paste this link into the address bar, and press Enter. It's a direct download for the free version of Malwarebytes' Anti-Malware (MBAM). Install it, get updates and run a full scan (still in Safe Mode):
http://www.myantispyware.com/mbam
If the problem still exists, download and run the free Kaspersky Virus Removal Tool:
http://www.kaspersky.com/antivirus-removal-tool?form=1
You should now delete TDSSKiller.exe and the Kaspersky Virus Removal Tool, as updated versions are often made available. Malwarebytes' Anti-Malware can be easily uninstalled, should you wish to do so, but it may prove to be beneficial in the future.
After this, try rebooting normally. If that's successful, I recommend you run another full scan with MBAM. It will detect malware that wasn't running in Safe Mode.
Once you're infection-free..
This is the set-up I use on my PC:
I have the free version of Avast Antivirus, along with the free version of Malwarebytes' Anti-Malware (which you should now have) and the free Comodo firewall (Comodo Internet Security). If you decide to download Comodo, use the "Customize Installer" option mentioned in the techsupportalert link below, and untick the antivirus. You only want to install the firewall. It's advisable to adjust some Comodo settings, in order to avoid conflicts with Avast. I used the settings recommended here:
http://www.techsupportalert.com/content/how-install-comodo-firewall.htm
Here are the download links:
http://www.filehippo.com/download_avast_antivirus/
http://www.filehippo.com/download_comodo/
These links may prove informative:
http://www.avast.com/pr-avast-free-antivirus-outperformed-all-free-and-a-number-of-paid-for-antivirus-products-in-av-tests-latest-dynamic-test
http://www.av-test.org/en/tests/home-user/windows-xp/julaug-2012/
http://chart.av-comparatives.org/chart2.php
Hope this helps.