Windows users are being urged to download the latest security patches from Microsoft to protect themselves.
The April update includes a fix for a flaw in Vista
In its monthly security update Microsoft released five patches, four of which are for flaws the software firm rates as "critical".
Flaws given this rating are so serious that they could be used to hijack a Windows PC without a user's knowledge.
The update includes a new version of a patch released in early April that had been causing problems for some users.
The second Tuesday of every month is the day Microsoft chooses to issue security updates for its software.
The five updates issued in April are for users of Windows Vista, XP, 2000, Server 2003 and Microsoft Content Management Server.
The four critical patches are for flaws that Microsoft says could lead to "remote code execution" which means that hackers could exploit them to take over a PC, steal information on it or put it to their own uses.
Also in the security bundle is a fix for a patch that Microsoft released in early April to close a flaw in the way that Windows handles animated cursors.
It released the patch early because many criminal hacking groups were using the flaw to set up booby-trapped websites that could hijack a PC.
Among those targeted in this way were World of Warcraft players as a single account for the hugely popular game can prove very lucrative.
However, Microsoft was forced to issue a fix for the cursor patch as some users found it conflicted with other programs installed on their PC.
Users of programs such as ElsterFormular, TUGZip, CD-Tag, and Realtek HD Audio Control Panel found that installing the patch stopped these utilities working.