Your computer may have been compromised. Do not access anything sensitive until you've run these free online virus scanners:
http://www.bitdefender.com/scan8/ie.html
http://www.eset.com/onlinescan/
http://www.kaspersky.com/scanforvirus (Online version of Kaspersky doesn't remove anything, so you'll have to manually delete.)
http://us.trendmicro.com/us/products.../trialpay.html
Also download SUPERAntiSpyware, which is free and really good:
http://www.superantispyware.com/download.html