A rivalry between the creators of the Netsky and Bagle viruses helped cause a dramatic increase in threats against home and enterprise computers in the first half of this year, but the most serious threat was Download.Ject, a Trojan that exploited a vulnerability in Microsoft Internet Explorer, according to McAfee.

McAfee's Anti-virus and Vulnerability Emergency Response Team (AVERT) ranked Exploit-MhtRedir.gen, also known as Download.Ject or Scob, as the top threat because it was used in a high number of attacks against both enterprises and consumers, took advantage of the widely used Internet Explorer browser and was a new type of threat, said Vincent Gullotto, vice president of AVERT.

AVERT is today releasing a list of the 10 biggest malicious threats in the first half of this year. For the first time, the company looked at not just the prevalence of the threat in terms of reports from end users, but also special circumstances, Gullotto said. Those included whether the threat hit corporations, whether it represented a new approach and whether a patch was available for it. A war between virus writers, such as the Netsky-Bagle rivalry, is another factor.

About 60 percent of all the malicious threats tracked by AVERT are what McAfee calls Potentially Unwanted Programs, or PUPs, giving customers the chance to decide whether they want to keep the software.

These include "adware" and "spyware," which may even be legitimate software but end up on a system without the user's knowing consent, Gullotto said. Reports of PUPs are increasing both because the software is growing more prevalent and because McAfee has added more reporting capabilities for it, he said.

Here are McAfee's top 10 threats of the year so far:

1. Exploit-MhtRedir.gen (also known as Download.Ject or Scob)
2. VBS/Psyme
3. Adware-Gator
4. Adware-180Solutions
5. Adware-Cydoor
6. Adware-BetterInet
7. W32/[email protected]
8. W32/[email protected]
9. W32/[email protected]
10. W32/[email protected]